Native Gold in the Chudnoe Au-Pd-REE Deposit (Subpolar Urals, Russia): Composition, Minerals in Intergrowth and Genesis

Article Native Gold in the Chudnoe Au-Pd-REE Deposit (Subpolar Urals, Russia): Composition, Minerals in Intergrowth and Genesis

Galina Palyanova 1,2,* , Valery Murzin 3 , Andrey Borovikov 1, Nikolay Karmanov 1 and Sergei Kuznetsov 4

1 Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, Akademika Koptyuga Pr., 3, 630090 Novosibirsk, Russia; [email protected] (A.B.); [email protected] (N.K.) 2 Department of Geology and Geophysics, Novosibirsk State University, Pirogova Str., 2, 630090 Novosibirsk, Russia 3 Zavaritsky Institute of Geology and Geochemistry, Ural Branch of Russian Academy of Sciences, Akademika Vonsovskogo Str., 15, 620016 Ekaterinburg, Russia; [email protected] 4 Komi Science Center, Institute of Geology, Ural Branch, Russian Academy of Sciences, Pervomaiskaya Str., 54, 167982 Syktyvkar, Russia; [email protected] * Correspondence: [email protected]

Abstract: Composition of native gold and minerals in intergrowth of the Chudnoe Au-Pd-REE deposit (Subpolar Urals, Russia) was studied using optical microscopy, scanning electron microscopy, and electron microprobe analysis. Five varieties of native gold have been identified, based on the set of impurity elements and their quantities, and on intergrown minerals. Native gold in rhyolites from the Ludnaya ore zone is homogeneous and contains only Ag (fineness 720‰, type I). It is in intergrowth with fuchsite or allanite and mertieite-II. In rhyolites from the Slavnaya ore zone, native Citation: Palyanova, G.; Murzin, V.; gold is heterogeneous, has a higher fineness, different sets and contents of elements: Ag, Cu, 840– Borovikov, A.; Karmanov, N.; 860‰ (type II); Ag, Cu, Pd, 830–890‰ (III); Ag, Pd, Cu, Hg, 840–870‰ (IV). It occurs in intergrowth Kuznetsov, S. Native Gold in the with fuchsite, albite, and mertieite-II (type II), or albite, quartz, and atheneite (III), or quartz, albite, K- Chudnoe Au-Pd-REE Deposit feldspar, and mertieite-II (IV). High-fineness gold (930–1000‰, type V) with low contents of Ag, Cu, (Subpolar Urals, Russia): Composition, Minerals in Intergrowth and Pd or their absence occurs in the form as microveins, fringes and microinclusions in native gold and Genesis. Minerals 2021, 11, 451. II–IV. Tetra-auricupride (AuCu) is presented as isometric inclusions in native gold II and platelets https://doi.org/10.3390/min11050451 in the decay structures in native gold III and IV. The preliminary data of a fluid inclusions study showed that gold mineralization at the Chudnoe deposit could have been formed by chloride fluids Academic Editor: Theodore of low and medium salinity at temperatures from 105 to 230 ◦C and pressures from 5 to 115 MPa. J. Bornhorst The formation of native gold I is probably related to fuchsitization and allanitization of rhyolites. The formation of native gold II-V is also associated with the same processes, but it is more complicated Received: 20 February 2021 and occurred later with a significant role of Na-, Si-, and K-metasomatism. The presence of Pd and Accepted: 22 April 2021 Cu in the ores and Cr in fuchsite indicates the important role of mafic-ultramafic magmatism. Published: 25 April 2021

Keywords: Chudnoe deposit (Russia); Au-Pd-REE mineralization; chemistry of native gold; Au-Cu Publisher’s Note: MDPI stays neutral intermetallides; P,T,X parameters of ore-forming ﬂuids with regard to jurisdictional claims in published maps and institutional afﬁl- iations.

1. Introduction The Chudnoe deposit (Subpolar Urals, Russia) [1–3] and some other Ural deposits— Baronskoe [4], Volkovskoe, Nesterovskoe [1,3,5–7], and Ozernoe [8,9] are unique in the set Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. of impurity elements in native gold (Ag, Cu, Pd, Hg), variability of their concentrations, This article is an open access article and minerals in intergrowth with it. The Chudnoe deposit is a special type of Au-Pd-REE distributed under the terms and mineralization [1,3], which differs from the known hydrothermal or metamorphic gold conditions of the Creative Commons deposits. Native gold at this deposit is associated with chromium muscovite (fuchsite), REE Attribution (CC BY) license (https:// minerals, palladium arsenoantimonides in the absence of iron sulﬁdes and carbonates. It creativecommons.org/licenses/by/ was discovered by Ozerov in 1994 [10] on the eastern slope of the Maldynyrd ridge during 4.0/). prospecting works on primary deposits, which are probable sources of earlier discovered

Minerals 2021, 11, 451. https://doi.org/10.3390/min11050451 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 451 2 of 26

placers with gold and palladium minerals in the Kozhim district. Since that time, numerous papers have been published on the mineralogy of the Chudnoe deposit in the form of a number of publications in the proceedings of Russian conferences, one monograph [3], three Ph.D. theses [5,11,12], and about ten articles [1–3,7,13–16]. In the earlier works on the mineralogy of gold from the Chudnoe deposit the authors distinguish from one to three varieties of native gold [1–3,16]. Native gold contains major (>10 wt.%), minor (1–10 wt.%), and trace (<1 wt.%) elements. The first data on the composition of native gold were reported by Tarbaev and coauthors [1], revealing that gold particles are concentrated near fuchsite veinlets. The first variety is the products of decomposition of Ag-Cu-Hg-Pd solid solution. The matrix is depleted in Pd (0.3–0.8 wt.%) relative to the Au-Cu phase of the platelets (0.7–1.1 wt.% Pd) and has a fineness of 840–870‰ (NAu = Au × 1000/(Au + Ag + Cu + Hg + Pd)). The composition of platelets (1–2 µm thick and to 10 µm long) was determined approximately: maximum content of Cu is 10.5 wt.%, the amount of Hg is close to that in the matrix 0.1–1 wt.%, and Ag ranges from 2.5 to 11.6 wt.%. The second variety of native gold is represented by a higher fineness (to 1.9 wt.% Pd) porous gold in the form of isometric 3–15 µm inclusions in native gold of the first type or thin rims around it. Galankina et al. [2] found one variety of native gold with high concentrations of Ag (to 20–25, occasionally to 50.5 wt.%) and low concentrations of Cu and Pd (no more than 0.2 wt.%) (fineness 510–800‰) in the absence of Hg. Gold particles to 3 mm in size were concentrated in fuchsite-quartz-allanite veinlets and often in intergrowth with mertieite or contained its inclusions. Shumilov and Ostashchenko [3] revealed three varieties of that native gold from the Chudnoe deposit, which are associated with either fuchsite or allanite localized in the axial part of mica veinlets. Native gold with a wide range of impurities, to 2.3 wt.% Pd, to 3.6 wt.% Cu, and to 1.4 wt.% Hg (840–870‰), was attributed to the first type. It has an intricate inner structure: some of gold particles (840–870‰) are rimmed with high-fineness gold (920–990‰) only with an impurity of Cu, whereas other gold particles contained the decomposition products of solid solutions. The platelets are from 0.1 to 1–3 µm thick, the amount of Cu in the platelets is higher (increase to 12.5 instead of 3–4 wt.%), and that of Ag is lower than in the matrix (decreases from 11–12 to 3–4 wt.%). Native gold of the second type is characterized by a low fineness (720–790‰, as low as 530‰) and absence of Cu, Hg, and Pd. This type of native gold occurs in the Ludnaya zone. To the third type, these authors [3] attribute porous and microcrystalline native gold that overgrows on larger gold particles of the first and second types. It is characterized by high fineness (to 990‰) and, respectively, lower contents of Ag and Cu at relatively high concentrations of Pd and absence of Hg. Borisov [5] found that native gold from the Chudnoe deposit contained inclusions of the Au-Cu phases: Au3Cu, Au3Cu2, and Au2Cu with minor Pd. Kuznetsov et al. [8] also reported that native gold of heterogeneous composition (8.2–11.6 wt.% Ag, 1.3–3.0 wt.% Cu, about 1.3 wt.% Hg and to 1.7 wt.% Pd, fineness 850–906‰) contains inclusions of Au- Cu phases—tetra-auricupride AuCu and auricupride AuCu3 and Pd minerals—mertieite Pd5Sb2, isomertieite Pd5AsSb, atheneite (Pd,Hg)3As, stillwaterite Pd8As3, and stibiopalladinite Pd5Sb3. It is worth noting that the stoichiometric formulas of some minerals reported by these authors differ from those of minerals with similar names from the IMA database [17]: mertieite-I Pd5+x(Sb,As)2−x (x = 0.1−0.2), mertieite-II Pd8Sb2.5As0.5, omertieite Pd11Sb2As2, stibiopalladinite Pd5Sb2, atheneite Pd2(As0.75Hg0.25). Onishchenko et al. [16] determined two varieties of native gold in the Slavnaya ore zone at the Chudnoe deposit. The authors show a predominance of native gold containing 84–88 wt.% Au, 7–12 wt.% Ag, 1.3–5.5 wt.% Cu, 1–2 wt.% Pd and about 1 wt.% Hg. This native gold was found to be intergrown with small inclusions of high-fineness gold: 94–98 wt.% Au, 1.5–2 wt.% Pd, to 0.9 wt.% Cu and to 0.7 wt.% Ag. Minerals 2021, 11, 451 3 of 26

Despite the numerous results, the reasons of compositional changes in native gold and the presence of a wide set of impurity elements are not clear yet. Although native gold is known to be concentrated in the fuchsite veinlets in rhyolites, there is little information on matrix minerals in close growth with it and physicochemical conditions of the formation of Au-Pd-REE mineralization. The aim of our study is to reveal the specific features of the chemical composition of native gold and to determine the minerals in the intergrowth with it in the Slavnaya and Ludnaya ore zones of the Chudnoe deposit. The first results of the fluid inclusions study at the Chudnoe deposit were obtained by Surenkov [12]. This study provides limited new microthermometric analyses of fluid inclusions which are used to estimate the compositional features and PTX parameters for two ore zones. These data will help us to determine the possible sources of trace elements in native gold and to receive new information on the genesis of unique Chudnoe Au-Pd-REE deposit.

2. Geological Settings and Localizations The Chudnoe deposit occurs on the western slope of the Subpolar Urals (Russia) in the interformational contact zone of uralids (terrigeneous and sedimentary rocks O1–2) and preuralids (volcanogenic rocks of effusive and subvolcanic facies of acidic and basic composition R3-V) in the axial part of the Maldinskaya anticline. In the contact zone of the Precambrian and Ordovician rocks there are lenses of chloritoid-pyrophylite slates which are considered to be the relicts of metamorphosed weathering crusts of the Precambrian age [10]. Numerous dislocations with a break in continuity of mainly NE strike are developed in the area of the deposit. The largest of them is the Maldinskiy fault, a zone in which the Chudnoe deposit is localized [1,7]. Metasomatites and associated zones of pyritization, hydrothermal quartz veins, many of which are crystal-bearing, are widespread in the area. Occurrences of rare-earth, polymetallic, molybdenum, silver, and uranium-copper mineralization are present as well. Lying 1.5–2 km to the southeast of the Chudnoe deposit is the Nesterovskoe occurrence localized in the terrigenous formations of uralids (aleuroschists, sandstones and gritstone O1) with gold-platinoid-fuchsite mineralization. Gold-palladium mineralization formed much later than the host rocks and it is of Paleozoic age (250–260 Ma), according to the 39Ar-40Ar datings on fuchsite [13]. The northwestern part of the Chudnoe deposit is composed of rocks of basic composition rich in Na (Figure1). These rocks consist of metabasalts transformed into dark-gray schists and, to a lesser degree, of metadolerites and gabbrodolerites of more massive ap- pearance [7]. Alterations of rocks develop as chloritization, albitization, and epidotization. The accessory minerals of metabasic rocks are magnetite, ilmenite, titanite, and sulfides (pyrite, chalcopyrite). The content of gold in basaltoids is insignificant. The southeastern part of the deposit area is made up of mainly dark porphyric rhyolites with flow bandings. Along the dislocations with a break in continuity, rhyolites are often lightened. In addition, dark rhyolites contain lightened zones extending to 1 m, oriented according to fluidality. The dark color of rhyolites is due to the scattered fine (to 0.01 mm) impregnations of hematite, which is absent in light rocks [14]. The groundmass of rhyolites is composed of varying amounts of quartz, albite, K-feldspar, and sericite. The accessory minerals of rhyolites are ilmenite, titanite, allanite, apatite, zircon, monazite, and xenotime. As for dark rhyolites, lightened zones contain similar quantities of major components and low concentrations of Fe2O3 (Table1). Minerals 2021, 11, 451 4 of 26 Minerals 2021, 11, x FOR PEER REVIEW 4 of 26

Figure 1. 200‐km (inset), 8‐km (a), and 100‐m (b) scale maps of the Chudnoe deposit according to Figure 1. 200-km (inset), 8-km (a), and 100-m (b) scale maps of the Chudnoe deposit according to [7] [7] and photograph of ore sample (size 5 cm × 13 cm) from this deposit (c). 1—Kozhim Formation: andlimestones, photograph calcareous of ore shales sample and (size sandstones; 5 cm × 2—Saled13 cm) Formation: from this depositchlorite‐sericite (c). 1—Kozhim‐quartz shale, Formation: limestones,sandstone; 3—Obizskaya calcareous shales suite: and quartzite sandstones; sandstones, 2—Saled conglomerates, Formation: gravelstones; chlorite-sericite-quartz 4—Sable‐ shale, sandstone;gorskaya suite: 3—Obizskaya felsic and basic suite: effusive quartzite rocks, sandstones, tuffs, phyllitic conglomerates, schists; 5—Moroinskaya gravelstones; Formation 4—Sablegorskaya (R3mr): phyllitic schists, marbles, quartzites; 6—Khobein suite: quartzites, conglomerates, chlorite‐ suite: felsic and basic effusive rocks, tuffs, phyllitic schists; 5—Moroinskaya Formation (R mr): sericite‐quartz schists; 7—Puivinskaya suite: mica‐quartz schists, calcareous schists, interlayers of 3 phylliticquartzites, schists, marbles; marbles, 8—subvolcanic quartzites; rhyolites; 6—Khobein 9—granites; suite: 10—gabbro, quartzites, gabbro conglomerates,‐dolerites; chlorite-sericite-11—tec‐ quartztonic faults; schists; 12—Chudnoe 7—Puivinskaya (1) and suite: Nesterovskoe mica-quartz (2) deposits. schists, calcareous schists, interlayers of quartzites, marbles; 8—subvolcanic rhyolites; 9—granites; 10—gabbro, gabbro-dolerites; 11—tectonic faults; Table 1. Chemical12—Chudnoe composition (1) of andlight Nesterovskoe (L) and dark (D) (2) rhyolites deposits. from the Chudnoe deposit [14].

No. SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO СаО Na2O K2O P2O5 LOI Σ 41 L 77.32Table 1.0.17Chemical 11.63 composition 0.62 of0.50 light (L)0.02 and dark0.06 (D) rhyolites0.19 from1.99 the Chudnoe6.40 deposit0.02 [140.21]. 99.13 42 D 75.08 0.17 11.75 2.40 0.72 0.02 0.09 0.18 1.72 6.60 0.02 0.41 99.16 No.43 L SiO276.42 TiO0.172 Al212.14O3 Fe2O1.003 FeO0.65 MnO0.02 MgO0.10 0.16 CaO 1.85 Na 2OK6.60 2OP0.02 2O5 0.27 LOI99.40 Σ 41 L44 D 77.3273.56 0.170.19 11.6312.64 0.622.55 0.500.65 0.020.04 0.060.16 0.37 0.19 1.45 1.99 6.60 6.40 0.05 0.020.74 0.2199.00 99.13 42 D62 L 75.0875.44 0.170.19 11.7513.66 2.400.50 0.721.01 0.020.02 0.090.04 0.21 0.18 4.90 1.72 3.60 6.60 0.03 0.020.14 0.4199.74 99.16 63 D 73.06 0.19 14.29 1.49 0.79 0.02 0.06 0.23 4.80 3.80 0.04 0.38 99.15 43 L 76.42 0.17 12.14 1.00 0.65 0.02 0.10 0.16 1.85 6.60 0.02 0.27 99.40 64 L 74.94 0.19 13.91 0.30 0.58 0.02 0.01 0.22 5.70 3.10 0.02 0.05 99.04 44 D 73.56 0.19 12.64 2.55 0.65 0.04 0.16 0.37 1.45 6.60 0.05 0.74 99.00 65 D 75.34 0.19 13.15 1.86 0.65 0.02 0.05 0.23 5.00 3.20 0.02 0.21 99.92 62 L 75.44 0.19 13.66 0.50 1.01 0.02 0.04 0.21 4.90 3.60 0.03 0.14 99.74 21 L 77.76 0.17 12.17 0.13 0.60 0.01 0.04 0.15 2.78 5.70 0.02 0.18 99.71 63 D 73.06 0.19 14.29 1.49 0.79 0.02 0.06 0.23 4.80 3.80 0.04 0.38 99.15 22 D 75.36 0.18 12.68 1.78 0.43 0.01 0.06 0.14 2.90 5.30 0.02 0.27 99.13 64 L 74.94 0.19 13.91 0.30 0.58 0.02 0.01 0.22 5.70 3.10 0.02 0.05 99.04 71 L 73.42 0.19 14.20 0.21 0.86 0.01 0.06 0.17 4.50 5.70 0.03 0.15 99.53 65 D 75.34 0.19 13.15 1.86 0.65 0.02 0.05 0.23 5.00 3.20 0.02 0.21 99.92 72 D 72.00 0.19 14.54 2.41 0.29 0.02 0.05 0.16 5.60 4.70 0.02 0.17 100.15 21 L 77.76 0.17 12.17 0.13 0.60 0.01 0.04 0.15 2.78 5.70 0.02 0.18 99.71 4–2 L 78.92 0.10 10.80 0.37 1.22 0.01 0.10 0.18 3.67 3.40 0.02 0.6 99.39 22 D 75.36 0.18 12.68 1.78 0.43 0.01 0.06 0.14 2.90 5.30 0.02 0.27 99.13 4–1 D 75.67 0.12 12.11 1.59 1.08 0.01 0.12 0.20 4.41 3.65 0.02 0.4 99.38 71 L 73.42 0.19 14.20 0.21 0.86 0.01 0.06 0.17 4.50 5.70 0.03 0.15 99.53 72 D 72.00Notes: The 0.19 СО2 content 14.54 in the samples 2.41 does 0.29 not exceed 0.02 0.22 wt.%, 0.05 the F content 0.16 < 0.02 5.60 wt.%. LOI—loss 4.70 on 0.02 ignition. 0.17 100.15 4–2 L 78.92 0.10 10.80 0.37 1.22 0.01 0.10 0.18 3.67 3.40 0.02 0.6 99.39 4–1 D 75.67 0.12 12.11 1.59 1.08 0.01 0.12 0.20 4.41 3.65 0.02 0.4 99.38

Notes: The CO2 content in the samples does not exceed 0.22 wt.%, the F content < 0.02 wt.%. LOI—loss on ignition. MineralsMinerals2021 2021, ,11 11,, 451 x FOR PEER REVIEW 5 ofof 2626

OnishchenkoOnishchenko andand KuznetsovKuznetsov [[7]7] reportreport that that in in gold-bearing gold‐bearing rhyolites, rhyolites, the the content content of of CrCr reachesreaches 0.03–0.05 wt.%, wt.%, and and the the concentrations concentrations of of Au, Au, Ag Ag and and As As are are no nomore more than than few fewtens tens of ppm: of ppm: 25 Au, 25 3.4 Au, Ag 3.4 (which Ag (which corresponds corresponds to Au/Ag to Au/Ag ≈ 7), and≈ 7),6–21 and As. 6–21 The As.contents The contentsof some ofelements some elements amount amountto hundreds to hundreds of ppm: of 480–970 ppm: 480–970 Ba, 80–100 Ba, 80–100La, 160–230 La, 160–230 Ce, 60–110 Ce, 60–110Y, 30–40 Y, 30–40Nb, 180–260 Nb, 180–260 Zr. The Zr. total The totalcontents contents of REE of REEin rhyolites in rhyolites with with fuchsite fuchsite from from the theChudnoe Chudnoe deposit deposit average average 544 544 ppm ppm [5]. [ 5The]. The concentrations concentrations of ofHg Hg to to18 18 ppm ppm are are typical typical of offuchsite fuchsite ores, ores, barren barren rhyolites rhyolites contain contain up up to to18 18ppb ppb Hg Hg [18]. [18 In]. Ingold gold-bearing‐bearing rhyolites rhyolites the thecontent content of S of is S no is nomore more than than 0.01 0.01 wt.%. wt.%. Higher Higher sulfur sulfur contents contents (to (to 0.5 0.5 wt.%) wt.%) were were found found in insome some poorly poorly productive productive zones zones among among andesites andesites [7]. [7 ]. Au-Pd-REEAu‐Pd‐RЕЕ mineralizationmineralization occursoccurs inin severalseveral schistschist andand brecciatebrecciate zones zones 100 100 to to 460 460 m m inin lengthlength andand upup toto 60 60 m m in in thickness thickness in in rhyolites rhyolites (Figure (Figure1 ).1). The The ore ore zones zones have have a a steep steep (50–70(50–70°)◦) northwestern northwestern dip dip and and northeastern northeastern strike. strike. Au-Pd-REE Au‐Pd‐RЕЕ mineralizationmineralization in in the the schist schist andand brecciatedbrecciated zones occurs among fuchsite, fuchsite, quartz, quartz, and and quartz quartz-albite‐albite veinlets veinlets ranging ranging in inthickness thickness from from few few mm mm to to 1–1.5 1–1.5 cm. cm. The The Chudnoe deposit includesincludes thethe SlavnayaSlavnaya andand LyudnayaLyudnaya oreore zones (Figure 11).). The The main main zone zone is is the the Slavnaya Slavnaya located located in inthe the central central part partof the of deposit. the deposit. This zone This contains zone contains the richest the richest ores with ores highest with highestAu concentrations Au concentrations ~22 ppm ~22[7]. ppmA rather [7]. Asmall rather Ludnaya small ore Ludnaya zone is ore localized zone is in localized schistose in rhyolites schistose at rhyolitesthe contact at with the contactmafic rocks with (andesites maﬁc rocks and (andesites basalts). and It occurs basalts). to Itthe occurs southwest to the of southwest the Slavnaya of the ore Slavnaya zone at orea distance zone at of a distance200–250 m of and 200–250 rises m for and 50 m rises above for 50the m relief above [3]. the The relief Ludnaya [3]. The ore Ludnaya zone has orea lentiform zone has‐discontinuous a lentiform-discontinuous structure and structure contains and rich contains ores with rich visible ores with native visible gold. native The gold.content The of content noble metals of noble is nonuniform—from metals is nonuniform—from few to hundreds few to hundreds ppm. ppm.

3.3. MaterialsMaterials andand MethodsMethods SamplesSamples ofof Au-bearingAu‐bearing rhyolitesrhyolites collectedcollected fromfrom explorationexploration trenchestrenches werewere used. used. Nu-Nu‐ merous polished and thin sections were made of them. For our study we selected typical merous polished and thin sections were made of them. For our study we selected typical samples of rhyolite from the ore zones Ludnaya (No.1a—1154-6; No.1b—1154-6b) (Figure2a) samples of rhyolite from the ore zones Ludnaya (No.1a—1154‐6; No.1b—1154‐6b) (Figure and Slavnaya (No.2—1122-5, No.3—1122-12, No.4a—1122-1, No.4b—1122-1b; No.5—1122-13) 2а) and Slavnaya (No.2—1122‐5, No.3—1122‐12, No.4a—1122‐1, No.4b—1122‐1b; No.5— (Figures 6a–8a), in which native gold was found visually or under a microscope. 1122‐13) (Figures 6а–8a), in which native gold was found visually or under a microscope.

FigureFigure 2. 2. MacrophotographMacrophotograph of of a a polished polished section section of of rhyolite rhyolite from from the the Ludnaya Ludnaya zone zone ( (aa)) and and reﬂected reflected light microscopy image (b) and BSE (c) micrograph of its fragment (p.1) with a fuchsite (Cr‐Ms) light microscopy image (b) and BSE (c) micrograph of its fragment (p.1) with a fuchsite (Cr-Ms) veinlet containing native gold of type I (AuI), allanite (Aln), and mertieite (Mert). veinlet containing native gold of type I (AuI), allanite (Aln), and mertieite (Mert).

Minerals 2021, 11, 451 6 of 26

A macro- and microscopic study of the ore samples was conducted. Mineralogical research was carried out using an optical microscope Olympus BX51 (in the Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences (IGM SB RAS) (Novosibirsk, Russia). Chemical analyses of minerals were conducted at the Analytical Center for Multi-elemental and Isotope Research in the IGM SB RAS (Novosibirsk, Russia) by electron probe microanalysis (EPMA) using a MIRA 3 LMU scanning electron microscope (Tescan Orsay Holding, Brno, Czech Republic) equipped with an X-ray energy dispersive spectrometer (EDS) AZtec Energy XMax-50 (Oxford Instruments Nanoanalysis, Oxford, UK) (analysts Dr. N. Karmanov, M. Khlestov). The composition of native gold and accessory minerals was studied at the following parameters: accelerating voltage was 20 kV, live spectrum acquisition time was 60 sec (total area of spectra ~106 counts). The following X-rays were selected: K series for Fe, Cu, As and L series for Pd, Ag, Sb, Au, Hg. We used pure metals (Fe, Cu, Pd, Ag, Au) and InAs for As and HgTe for Hg as the standards. The detection limits (in wt.%) were: 0.1 Fe, 0.15 Cu, 0.25 Pd, Ag, Sb, 0.3 As, 0.6 Au, and 0.8 Hg. Error in determining the main components with the contents higher than 10 wt.% did not exceed 1 relative (rel.) %, and when the content of components ranged 2–10 wt.%, the error was no higher than 6–8 rel. %. Close to the limit of detection, the error was 15–20 rel. %. In some cases the spectrum acquisition time increased to 120 s, the lower limits of determined contents and the random error of the analysis decreased about 1.4 times. To reduce the effect of microrelief of samples on the quality of analysis, data on the primary homogeneous gold were obtained in the scanning mode of individual sections from 10 × 10 to 50 × 50 µm2 in size. In the same mode, we obtained the average composition of native gold in the presence of decay structures. The compositions of small gold particles (<10 µm) and platelets of decay structure were determined with a 10 nm point probe, but the size of the generation region of X-ray emission in gold with the electron beam energy 20 kV was 1 µm. Therefore, the data of analysis cannot be considered quantitative if the minimum size of the studied object is less than 2 µm. It is worth noting that in the obtained back scattered-electron (BSE) micrographs, albite and quartz as well as K-feldspar and muscovite (fuchsite) are hardly distinguishable when they are intergrown with each other. For better visualization of the matrix minerals, we used multi-layered colorful EDS maps. Fluid inclusion microthermometry was used to analyze the composition of fluids and to estimate the pressure-temperature conditions during ore forming processes (fluid inclusion study was carried out in the Laboratory of Prediction-Metallogenic studies, IGM SB RAS, Novosibirsk, Russia). Phase transition temperatures in fluid inclusions (samples No.1b—1154-6b; No.4b—1122-1b) were determined using cryo- and thermometry proce- dures (Linkam THMSG-600 heating-freezing chamber with the measurement range of –196 to +600 ◦C). According to the data of cryometry, the fluids are attributed to the water-salt system [19,20]. To estimate the pressure and other parameters of mineral formation from the microthermometric data of fluid inclusion study, we used the AqSo_NaCl software [21].

4. Results Detailed mineralogical studies reveal the varieties of native gold and the minerals in the intergrowths with it from the Ludnaya and Slavnaya ore zones of the Chudnoe Au-Pd- REE deposit. Native gold is intergrown with one up to ﬁve minerals. The intergrowths of native gold with minerals are common. In the subsections below, we describe the features of the revealed varieties of native gold and minerals in the intergrowths with it from two ore zones.

4.1. Native Gold in Rhyolites from the Ludnaya Ore Zone Figure2 shows the macrophotograph of a polished section of typical rhyolite from the Ludnaya zone (a) and micrographs (b,c) of its fragment (p.1) with a fuchsite veinlet containing native gold. The sizes of gold particles vary from barely discernible 0.1 to 70–100 µm. Small gold particles are virtually of isometric shape, occasionally with elements Minerals 2021, 11, x FOR PEER REVIEW 7 of 26

Figure 2 shows the macrophotograph of a polished section of typical rhyolite from Minerals 2021, 11, 451 7 of 26 the Ludnaya zone (а) and micrographs (b,c) of its fragment (p.1) with a fuchsite veinlet containing native gold. The sizes of gold particles vary from barely discernible 0.1 to 70– 100 μm. Small gold particles are virtually of isometric shape, occasionally with elements ofof faceting, faceting, whereas whereas larger larger individuals individuals are are of elongated of elongated interstitial interstitial shape. shape. Figures Figures3–5 show 3–5 theshow reﬂected the reflected light microscopy light microscopy image image (Figures (Figures3a and 43aa), and BSE 4a), micrographs BSE micrographs(Figure (Figure5a,b), multi-layered5a,b), multi‐layered colorful colorful EDS mapsEDS maps (Figures (Figures3b and 3b4 andb), and4b), and maps maps of areal of areal distribution distribution ofof elementselements inin characteristiccharacteristic raysrays (Figures(Figures3 c–i3c–i and and4c–g) 4c–g) of of fragments fragments (p.1a, (p.1a, p.1b) p.1b) with with goldgold particlesparticles ofof variousvarious morphologiesmorphologies andand sizessizes (Figure (Figure2 b).2b). The The gold gold particles particles have have a a homogeneoushomogeneous texture, texture, which which is is clearly clearly seen seen from from these these ﬁgures. figures.

FigureFigure 3.3. NativeNative gold gold (type (type I) I) (AuI) (AuI) (fineness (ﬁneness 720‰, 720‰ Ag, Ag impurity) impurity) in intergrowth in intergrowth with with fuchsite fuchsite (Cr‐ (Cr-Ms)Ms) in rhyolite in rhyolite from from the the Ludnaya Ludnaya zone zone (Figure (Figure 2b—p.1b).2b—p.1b). ( aa)—reflected)—reﬂected light light microscopy image,image, ((bb)—multi-layered)—multi‐layered colorful colorful EDS EDS map, map, ( c(c––i)—mapsi)—maps of of areal areal distribution distribution of of elements elements in in characteristic characteristic rays. Abbreviations of minerals: quartz (Qz), albite (Ab), K‐feldspar (Kfs). rays. Abbreviations of minerals: quartz (Qz), albite (Ab), K-feldspar (Kfs).

ItIt is is well well seen seen on on the the multi-layered multi‐layered colorful colorful EDS EDS maps maps (Figures (Figures3 b3b and and4b) 4b) and and maps maps ofof arealareal distributiondistribution ofof elementselements (Au,(Au, Ag,Ag, Pd,Pd, Na,Na, K,K, Si,Si, Al, Al, Fe, Fe, Sb, Sb, As) As) in in characteristic characteristic raysrays (Figures(Figures3 c–i3с–i and and4c–g) 4с–g) that that native native gold gold in in the the analyzed analyzed fragments—points fragments—points p.1a p.1a and and p.1b—isp.1b—is localizedlocalized inin thethe intersticesinterstices ofof fuchsitefuchsite andand hashas nono commoncommon bordersborders withwith quartz,quartz, albite,albite, andand K-feldspar.K‐feldspar. FigureFigure4 4demonstrates demonstrates that that native native gold gold (p.1a) (p.1a) is is intergrown intergrown with with fuchsitefuchsite and and mertieite. mertieite. MertieiteMertieite alsoalso occurs occurs as as separate separate particles particles in in fuchsite fuchsite (Figure (Figure4 a).4а).

Minerals 2021, 11, 451 8 of 26 MineralsMinerals 2021 2021, 11, 11, x, xFOR FOR PEER PEER REVIEW REVIEW 8 8of of 26 26

FigureFigure 4. 4. NativeNative Native gold gold gold (type (type (type I) I) I) (AuI) (AuI) (AuI) (fineness (ﬁneness (fineness 720‰, 720 720‰,‰, Ag Ag Ag impurity) impurity) impurity) in in in intergrowth intergrowth intergrowth with with with fuchsite fuchsite fuchsite (Cr (Cr- (Cr‐ ‐ Ms)Ms) and and mertieite mertieite (Mert) (Mert) ( b(b––dd) )in in rhyolite rhyolite from from the the Ludnaya Ludnaya zone zone (Figure (Figure2 2b—p.1a).b—p.1a). 2b—p.1a). ( ( a(a)—reﬂected)—reflected)—reflected lightlightlight microscopy microscopy image, image, ( b(b)—multi)—multi-layered)—multi‐layered‐layered colorful colorful EDS EDS map, map, ( c(–c–gg)—maps)—maps of of areal areal distribution distribution of of elementselements in in characteristic characteristic rays. rays.

FigureFigure 55a,b a,b5a,b showsshows shows relativelyrelatively relatively smallsmall small particlesparticles particles ofof of nativenative native goldgold gold (type(type (type I)I) I) (Figure(Figure (Figure2 2b—p.1b—p.1c), 2b—p.1сс),), surroundedsurrounded by by fuchsite fuchsite or or allanite. allanite. Allanite AllaniteAllanite is isis localized localizedlocalized in inin the the axial axial part part of of the the fuchsite fuchsite veinletveinlet (Figures (Figures 22 c2с с andandand5 5a). 5аа).). It It It is is is seen seen seen from from from Figure Figure Figure5 b5b 5b that that that one one one of of of the the the gold gold gold particles particles particles is is inter-is inter inter‐‐ growngrown with with mertieite mertieite and and allanite, allanite, and and between between allanite allanite and and fuchsite fuchsite there there are are aggregates aggregates ofof quartz quartz and and K K-feldspar, K‐feldspar,‐feldspar, which which also also proves proves the the absence absence of of joint joint boundaries boundaries of of native native gold gold (type(type 1) 1) with with these these minerals minerals in in rhyolite. rhyolite.

FigureFigure 5. 5. BSE BSE micrographs micrographs of of a a fuchsite fuchsite (Cr (Cr-Ms) (Cr‐Ms)‐Ms) vein vein with with allanite allanite (Aln), (Aln), native native gold gold (type (type I) I) (AuI) (AuI) (Figure(Figure2 2b—p.1c)b—p.1c) 2b—p.1c) and and and other other other minerals minerals minerals in in rhyolitein rhyolite rhyolite from from from the the Ludnayathe Ludnaya Ludnaya zone zone zone ( a) and( a(a) )and itsand enlarged its its enlarged enlarged fragment frag frag‐ ‐ ment (b): on the right are gold microparticles in intergrowth with fuchsite, on the left, in intergrowth ment(b): on (b the): on right the right are gold are microparticlesgold microparticles in intergrowth in intergrowth with fuchsite,with fuchsite, on the on left, the in left, intergrowth in intergrowth with with allanite (Aln) and mertieite (Mert). Abbreviations of minerals: quartz (Qz), K‐feldspar (Kfs). withallanite allanite (Aln) (Aln) and mertieite and mertieite (Mert). (Mert). Abbreviations Abbreviations of minerals: of minerals: quartz quartz (Qz), (Qz), K-feldspar K‐feldspar (Kfs). (Kfs).

NativeNative gold goldgold (type (type I) I) I) in in in intergrowth intergrowth intergrowth with with with fuchsite, fuchsite, fuchsite, allanite, allanite, allanite, and and andmertieite mertieite mertieite in in green green in green vein vein‐‐ letsveinletslets of of rhyolite rhyolite of rhyolite sample sample sample No.1 No.1 from No.1 from the from the Ludnaya Ludnaya the Ludnaya ore ore zone zone ore (Figure (Figure zone (Figure 2a) 2a) has has2 aa) a constant constant has a constant compo compo‐‐ sition and major Ag impurity (fineness 716–724‰, NAu = Au * 1000/(Au + Ag)) (Table 2). sitioncomposition and major and Ag major impurity Ag impurity (fineness (ﬁneness 716–724‰, 716–724 NAu ‰= Au,N Au* 1000/(Au= Au * 1000/(Au + Ag)) (Table + Ag)) 2).

Minerals 2021, 11, 451 9 of 26

(Table2). It is worth noting that Pd, Cu, Hg, and other elements were not detected even when the acquisition time of the spectra was increased to 120 s.

Table 2. Representative analyses of native gold (type I) and minerals in intergrowth with it in rhyolite No.1 (Figures2–5) from the Ludnaya ore zone (EPMA data in wt.% and formula).

N Au Ag ∑ wt.% NAu Formula Minerals in Intergrowth 69.54 27.6 97.14 716 Au Ag Cr-Ms; U-Mert p.1a 0.58 0.42 70.52 27.22 97.74 722 Au0.59Ag0.41 -“- 71.44 27.29 98.72 717 Au Ag Cr-Ms p.1b 0.58 0.42 70.52 27.22 97.74 724 Au0.59Ag0.41 -“- 69.91 27.12 97.03 720 Au Ag Cr-Ms p.1c 0.59 0.41 69.95 27.46 97.41 722 Au0.59Ag0.41 Aln; Cu-Mert Abbreviations of minerals: fuchsite (Cr-Ms), allanite (Aln) and U or Cu-mertieite (U- or Cu-Mert).

Fuchsite from the Ludnaya ore zone, in addition to 0.4–1.1 Cr, also contains 6.4–6.5 Fe, 0.3–0.6 Mg, and 0.2–0.3 Ti (in wt.%). Its stoichiometric formula corresponds to K0.98Fe0.52 Ti0.02Cr0.01Mg0.09Al2.24Si3.24O11H4.5. Allanite has a varying composition with impurities of (in wt.%): 0.1–0.3 Ce and Nd, up to 0.5 Pr, up to 0.1 La, Sm, Cr, and Mn (Ca3.2–3.8Fe2–2.1La0–0.1 Ce0–0.3Nd0–0.3Pr0–0.1Mn0.1Cr0–0.1Al3.9–4.1Si6.2O25H1.9). In some allanite crystals, we observed REE together with about 1 wt.% Sr, which corresponds to (Ca4Fe2.1Sr0.1La0.1Ce0.3 Nd0.3Pr0.1Sm0.1Al4.1Si6.2O25H1.9). The stoichiometric formula of palladium arsenoantimonide, according to IMA data, is similar to mertieite-II (Pd8Sb2.5As0.5). The difference between mertieite-I and mertieite-II is chemical: mertieite-I Pd5+x(Sb,As)2−x (x = 0.1 − 0.2), mertieite-II Pd8Sb2.5As0.5. The stoichiometric formula of this phase in the fuchsite matrix is Pd7.7Sb2.6As0.6U0.1 (content of U impurity is about 2 wt.%), and in the intergrowth with allanite it revealed higher contents of As, presence of Cu, and absence of U: Pd7.5Cu0.5Sb1.6As1.4. Fuchsite veinlets also contain titanite CaTiSiO6 and Y mineral, which is similar in composition to keiviite (Y2Si2O7). Rhyolite, in addition to quartz, albite, and K-feldspar, contains hematite (Figure2a), less frequently, zircon and apatite. In zircon, hafnium (1–1.4 wt.%) was present. Titanite was found to contain (in wt.%): 1–1.2 Nb, 0.3 Cr, and 0.9 Fe. Results of studies showed that native gold of type I occurs in the form of individual particles or in intergrowth with mertieite-II in the allanite-fuchsite matrix in rhyolite from the Ludnaya ore zone and has direct contacts with these minerals, which suggests simultaneous formation of Au-Pd-REE mineralization and fuchsite veinlets and, probably, similar physicochemical conditions of deposition.

4.2. Native Gold in Rhyolites from the Slavnaya Ore Zone Native gold in rhyolites from the Slavnaya ore zone, in contrast to the Ludnaya ore zone, along with silver contains copper, palladium, and mercury in various combinations and concentrations. The fineness of native gold (NAu, ‰) was calculated using the equation: Au (in wt.%) * 1000/(Au + Ag + Cu + Pd + Hg) (in wt.%). Depending on the set and contents of impurity elements in native gold, its reference symbols were adopted-Au(NAu)AgCuPdHg (elements are given in order of decreasing concentrations). Several varieties of native gold were established (Tables3–6): (1) Native gold with major content of Ag and minor Cu (fineness 860‰) (type II) (AuII860AgCu); (2) native gold with major Ag and insignificant amounts of Cu and Pd (fineness 830–890‰) (type III) (AuIII830–890AgCuPd); (3) native gold with major Ag and insignificant amounts of Cu, Pd, and Hg (fineness 830–860‰) (type IV) (AuIV830–860AgCuPdHg); (4) high-fineness gold (930–1000‰) with insignificant amounts of Ag, Cu, and (or) Pd (type V) (AuV930AgCuPd-AuV), the total content of which does not exceed 7 wt.%, or pure gold (AuV). We found Au-Cu intermetallides in association with native gold of types II–IV. Results of study of the composition and amount of impurity elements in native gold and minerals in intergrowth with it in rhyolites from the Slavnaya ore zone are reported in the subsections below in the order of revealed varieties. Minerals 2021, 11, 451 10 of 26

4.2.1. Native Gold with Ag and Cu Impurities Native gold containing major Ag and minor Cu (type II) was identiﬁed in two rhyolite samples from the Slavnaya ore zone No.2 (p.1a, 2, 6, 7) (Figure6) and No.3 (p.1b, 1c, 3b, 3c, 4a) (Figure7). Table3 shows typical compositions of this type of native gold and provides a list of intergrown minerals. In sample No.2 this native gold is localized in albite veins accompanying fuchsite veins (Figure6a–c). It is intergrown with both minerals albite and fuchsite (Figure6c,e,g,h) as well as with mertieite (Figure6d–g) and allanite (Figure6g,h). As an example, Figure6b–j shows optical and BSE micrographs of xenomorphic particles of native gold (type II) (20–40 µm in size) in intergrowth with albite, fuchsite, and mertieite (sample No.2, p.1a,6,7). The particles contain 11.4–12.7 wt.% Ag and 1.5–1.8 wt.% Cu, which corresponds to the ﬁneness 860–865‰ (Table3).

Table 3. Representative analyses of native gold (type II) and minerals in intergrowth with it in rhyolites from the Slavnaya ore zone (Nos. 2, 3, Figures6 and7) (EPMA data in wt.% and formula).

N Au Ag Cu ∑wt.% NAu Formula Minerals in Intergrowth No.2

p.1a 85.03 11.58 1.72 98.34 865 Au0.76Ag0.19Cu0.05 Cr-Ms; Ab; Cu,Ag-Mert p.1a 84.64 12.41 1.65 98.7 858 Au0.76Ag0.19Cu0.05 -“- Cr-Ms; Ab; Kfs; AuCu; p.2 84.55 11.51 4.12 100.19 844 Au0.71Ag0.18Cu0.11 AuIII840–890 p.2 84.76 12.72 1.55 99.03 856 Au0.75Ag0.21Cu0.04 -“- p.6 83.68 12.24 1.61 97.53 858 Au0.75Ag0.20Cu0.04 Cr-Ms; Ab; Aln; AuV930–990 p.6 83.78 12.19 1.77 97.73 857 Au0.75Ag0.20Cu0.05 -“- Cr-Ms; Ab; Cu,Ag,Au-Mert; p.7 85.38 11.44 1.48 98.3 869 Au0.77Ag0.19Cu0.04 AuV990 p.7 85.73 11.87 1.62 99.23 864 Au0.77Ag0.19Cu0.04 Cr-Ms; Ab; AuV990 p.7 85.67 12.28 1.54 99.48 861 Au0.76Ag0.20Cu0.04 -“- p.7 84.41 12.38 1.4 98.19 860 Au0.76Ag0.20Cu0.04 Ms; Ab; Aln; AuV990 No.3 Ab; Ms; Kfs; Qz; AuV ; p.1b 85.22 11.86 1.88 98.96 861 Au Ag Cu 1000–970 0.76 0.19 0.05 AuCu; X1 p.1b 84.93 11.95 2.04 98.92 859 Au0.75Ag0.19Cu0.06 -“- p.1b 84.21 12.57 1.51 98.29 857 Au0.75Ag0.21Cu0.04 -“- p.1e 83.62 11.75 4.64 100.01 836 Au0.70Ag0.18Cu0.12 Ab; Kfs; Qz; AuIII840–890 p.2 86.84 12.83 2.24 101.9 852 Au0.74Ag0.20Cu0.06 Ab; Ms; Qz p.2 84.57 12.9 2.11 99.58 849 Au0.74Ag0.21Cu0.05 -“- p.2 86.36 12.27 1.71 100.35 861 Au0.76Ag0.20Cu0.05 -“- p.2 85.39 12.98 2.02 100.39 851 Au0.75Ag0.21Cu0.06 -“- p.2 83.16 14.85 1.35 99.36 837 Au0.73Ag0.23Cu0.04 -“- p.2 84.86 12.29 2.42 99.57 852 Au0.73Ag0.20Cu0.07 -“- p.3b 84.13 12.21 1.67 98.01 858 Au0.75Ag0.2Cu0.05 Ab; Ms p.3c 84.82 12.19 1.76 98.77 859 Au0.75Ag0.2Cu0.05 Ab p.4a 84.54 12.33 1.93 98.81 856 Au0.75Ag0.2Cu0.05 Ab; Kfs; Cu-Mert; X1 p.4a 84.07 12.34 2.07 98.48 854 Au0.74Ag0.2Cu0.06 -“- Abbreviations of minerals: native gold of type II, III and V (AuII, AuIII and AuV), fuchsite (Cr-Ms), muscovite (Ms), allanite (Aln), quartz (Qz), albite (Ab), K-feldspar (Kfs) Cu, Ag, U-mertieite (Cu, Ag, U-Mert), tetra-auricupride (AuCu) and amoeba phase (X1).

Figure6g shows native gold of type II in the intergrowth with allanite, albite, fuchsite, and mertieite in the same sample No.2 (p.7). The contact between native gold and allanite is not resorbed, which suggests that they formed simultaneously. In some albite-fuchsite veinlets of sample No.2 (p.6) (Figure6h–j) gold particles in the marginal parts are replaced by higher-ﬁneness gold (ﬁneness 930–965‰) (type V): the content of Ag decreases to 2.2 wt.%, and that of Cu increases to 2.3 wt.% and Pd appears (0.9–1.9 wt.%) (Table6). The texture of such native gold becomes porous (Figure6j). The albite-fuchsite-allanite veinlet contains apatite inclusions (Ca4.9P2.82O12F1) (Figure6h). Minerals 2021, 11, x FOR PEER REVIEW 11 of 26

Figure 6g shows native gold of type II in the intergrowth with allanite, albite, fuch‐ site, and mertieite in the same sample No.2 (p.7). The contact between native gold and allanite is not resorbed, which suggests that they formed simultaneously. In some albite‐ fuchsite veinlets of sample No.2 (p.6) (Figure 6h–j) gold particles in the marginal parts are replaced by higher‐fineness gold (fineness 930–965‰) (type V): the content of Ag de‐ Minerals 2021, 11, 451 creases to 2.2 wt.%, and that of Cu increases to 2.3 wt.% and Pd appears (0.9–1.911 wt.%) of 26 (Table 6). The texture of such native gold becomes porous (Figure 6j). The albite‐fuchsite‐ allanite veinlet contains apatite inclusions (Ca4.9P2.82O12F1) (Figure 6h).

Figure 6. Macrophotograph of a polished section of rhyolite No. 2 from the Slavnaya ore zone (a) Figure 6. Macrophotograph of a polished section of rhyolite No.2 from the Slavnaya ore zone (a) with with albite‐fuchsite veins and its fragments with native gold (types II, V) (p.1,2,6,7) (b–j). (b,d,f,i) albite-fuchsite veins and its fragments with native gold (types II, V) (p.1,2,6,7) (b–j). (b,d,f,i) Reﬂected Reflected light microscopy image; (c,e,g,h,j) BSE micrographs. (j) Enlarged fragment on h. Abbre‐ lightviations microscopy of minerals: image; native (c,e ,goldg,h,j )(type BSE micrographs.II, V) (AuII, AuV), (j) Enlarged fuchsite fragment (Cr‐Ms), allanite on h. Abbreviations (Aln), albite of minerals:(Ab), apatite native (Apt), gold mertieite (type II, (Mert). V) (AuII, AuV), fuchsite (Cr-Ms), allanite (Aln), albite (Ab), apatite (Apt), mertieite (Mert).

In sample No.3 (Figure7a), native gold of type II is concentrated in albite veinlets with

rare fragments of fuchsite and it is intergrown with albite (p.3c) or albite and fuchsite (p.3b) (Figure7b) or albite, fuchsite, and mertieite (p.4a) (Figure7c) or albite, fuchsite, quartz, and K-feldspar (p.1b,1c) (Figure7d,f). It is localized in the monomineral albite matrix (sample No.3, p.3c) or bimineral albite-fuchsite matrix (sample No.3, p.3b) (Figure7b), it is homogeneous and contains 11.5–12.8 wt.% Ag and 1.5–2.4 wt.% Cu, its ﬁneness is 860‰ (Table3). Minerals 2021, 11, x FOR PEER REVIEW 12 of 26

In sample No.3 (Figure 7а), native gold of type II is concentrated in albite veinlets with rare fragments of fuchsite and it is intergrown with albite (p.3с) or albite and fuchsite (p.3b) (Figure 7b) or albite, fuchsite, and mertieite (p.4a) (Figure 7c) or albite, fuchsite, quartz, and K‐feldspar (p.1b,1c) (Figure 7d,f). It is localized in the monomineral albite ma‐ Minerals 2021, 11, 451 trix (sample No.3, p.3с) or bimineral albite‐fuchsite matrix (sample No.3, p.3b) (Figure12 of 7b), 26 it is homogeneous and contains 11.5–12.8 wt.% Ag and 1.5–2.4 wt.% Cu, its fineness is 860‰ (Table 3).

FigureFigure 7. 7.Macrophotograph Macrophotograph of of a a polished polished section section of of rhyolite rhyolite No.3 No.3 from from the the Slavnaya Slavnaya ore ore zone zone ( a(),a), multi-layeredmulti‐layered colorful colorful EDS EDS maps maps of of its its fragments fragments (p.3,4,1) (p.3,4,1) (b (–bd–),d), reﬂected reflected light light microscopy microscopy image image (e,g) and BSE micrographs (f,h) of native gold (types II, III, V, AuCu) in intergrowth with quartz (e,g) and BSE micrographs (f,h) of native gold (types II, III, V, AuCu) in intergrowth with quartz (Qz), (Qz), albite (Ab), fuchsite (Cr‐Ms), K‐feldspar (Kfs), amoeba phase (X1) (f) (p.1b) or with quartz, albite (Ab), fuchsite (Cr-Ms), K-feldspar (Kfs), amoeba phase (X1) (f) (p.1b) or with quartz, albite, albite, atheneite (Atn) and amoeba phase (X2) (h) (p.1a). Abbreviations of minerals: native gold (type atheneiteII, III, V) (Atn) (AuII, and AuIII, amoeba AuV), phase tetra (X2)‐auricupride (h) (p.1a). (AuCu). Abbreviations of minerals: native gold (type II, III, V) (AuII, AuIII, AuV), tetra-auricupride (AuCu).

Native gold of type II in the polymineral matrix p.1b (sample No.3) (Figure7d), consisting of albite, fuchsite, quartz, and K-feldspar, has a heterogenous texture (Figure7f) . It contains isometric inclusions (to 10 × 12 µm2 in size) of tetra-aucupride with trace concentrations of Pd (0.9 wt.%) (Au1.07Cu0.91Pd0.02) (III type) (Table7), and microinclusions of higher-ﬁneness (ﬁneness 980‰, with trace contents of Cu, Ag, no Pd) or pure gold Minerals 2021, 11, x FOR PEER REVIEW 13 of 26

Native gold of type II in the polymineral matrix p.1b (sample No.3) (Figure 7d), con‐ sisting of albite, fuchsite, quartz, and K‐feldspar, has a heterogenous texture (Figure 7f). Minerals 11 2021, , 451 It contains isometric inclusions (to 10 × 12 μm2 in size) of tetra‐aucupride with trace13 ofcon 26‐ centrations of Pd (0.9 wt.%) (Au1.07Cu0.91Pd0.02) (III type) (Table 7), and microinclusions of higher‐fineness (fineness 980‰, with trace contents of Cu, Ag, no Pd) or pure gold (fine‐ (ﬁnenessness 1000‰, 1000 without‰, without impurities) impurities) (type (type V) (Table V) (Table 6).6 In). Inmost most gold gold particles particles the the content content of ofCu Cu is is1.9–2.0 1.9–2.0 wt.% wt.% and and that that of of Ag Ag is is 11.9–12.6 11.9–12.6 wt.%, wt.%, which which corresponds corresponds to to fineness ﬁneness 860860‰.‰. InIn thethe marginalmarginal partsparts ofof thesethese goldgold particles particles one one can can observe observe amoeboid-like amoeboid‐like rims. rims. TheThe chemicalchemical compositioncomposition of these rims is similarsimilar to thethe mixturemixture ofof albitealbite withwith thethe Х X11 phase,phase, containing,containing, alongalong withwith mainmain elementselements Na,Na, Al,Al, Si,Si, andand O,O, impuritiesimpurities ofof YY (2.1),(2.1), ZrZr (2.8),(2.8), CeCe (0.6),(0.6), UU (1.3),(1.3), FeFe (0.2),(0.2), andand AsAs (0.9)(0.9) (in(in wt.%)wt.%) (Figure (Figure7 f,7f, p.1b). p.1b). TheThe stoichiometricstoichiometric formula formula of of palladium palladium arsenoantimonide, arsenoantimonide, according according to to IMA IMA data, data, is similaris similar to mertieite-II,to mertieite‐ butII, but contains contains 5.5 5.5 wt.% wt.% Cu Cu (No.3, (No.3, p.4a) p.4 orа) 5.1–5.4or 5.1–5.4 wt.% wt.% Cu Cu and and 0.7–0.9 0.7– wt.%0.9 wt.% Ag Ag (No.2, (No.2, p.1a), p.1а and), and occasionally occasionally Au Au too too (No.2, (No.2, p.7): p.7): 2.6–2.9 2.6–2.9 Cu; Cu; 0–0.8 0–0.8 Ag, Ag, 1.3–2.7 1.3–2.7 AuAu (in(in wt.%).wt.%). ThisThis phasephase hashas elementselements ofof faceting.faceting. TheThe boundariesboundaries betweenbetween thethe particlesparticles ofof mertieite-IImertieite‐II and native native gold gold of of type type II II are are straight. straight. Fuchsite Fuchsite contains contains the thefollowing following im‐ impuritiespurities (in (in wt.%): wt.%): 1.1–1.7 1.1–1.7 Cr, Cr, 4.6–5.7 4.6–5.7 Fe, Fe, 0.2 0.2 Ti, Ti, 0.5–0.8 0.5–0.8 Mg, Mg, 0.8 0.8 Mn, Mn, and and 0.3 0.3 Ba. Ba.

4.2.2.4.2.2. Native GoldGold withwith Ag,Ag, Cu,Cu, andand PdPd ImpuritiesImpurities NativeNative goldgold containing containing major major Ag, Ag, minor minor Cu, Cu, and and trace trace Pd Pd impurities impurities (type (type III) III)(Table (Table4 ) was4) was found found in rhyolitein rhyolite samples samples Nos. Nos. 3 (Figure 3 (Figure7a, p.1,7a, p.1, 3, 4) 3, and 4) and 4 (Figure 4 (Figure8a, p.2). 8a, p.2).

FigureFigure 8.8. MacrophotographMacrophotograph of of a a polished polished section section of of rhyolite rhyolite No.4 No. from4 from the the Slavnaya Slavnaya ore ore zone zone (a) ( anda) and multi‐layered colorful EDS maps of its fragments (p.2 and p.5) (b,c) with native gold (types multi-layered colorful EDS maps of its fragments (p.2 and p.5) (b,c) with native gold (types III—b, III—b, IV—c) in poly—(b) and bimineral (c) matrices: (b) Quartz, albite, K‐ feldspar, fuchsite; (c) IV—c) in poly—(b) and bimineral (c) matrices: (b) Quartz, albite, K- feldspar, fuchsite; (c) Quartz, Quartz, fuchsite. Abbreviations of minerals: native gold (type III, IV) (AuIII, AuIV), fuchsite (Cr‐ fuchsite.Ms), quartz Abbreviations (Qz), albite of(Ab), minerals: K‐feldspar native (Kfs). gold (type III, IV) (AuIII, AuIV), fuchsite (Cr-Ms), quartz (Qz), albite (Ab), K-feldspar (Kfs).

In sample No.3 (Figure7a), the minerals intergrown with gold particles are quartz, albite, and K-feldspar (p.3a, 4b—Figure7b,c), (p.1c—Figure7d–f), (p.1a—Figure7g,h). Na- tive gold of type III (grains of 50–70 µm in size), surrounded mainly by quartz, occasionally occurs in contact with albite and K-feldspar (sample No.3, p.1a) (Figure7d,h), contains (in Minerals 2021, 11, 451 14 of 26

wt.%): 12.8–13.1 Ag, 1.8–2.2 Cu, 0.7–1.0 Pd, and its ﬁneness varies in a narrow range of 840–860‰ (Table4).

Table 4. Representative analyses of native gold (type III) and minerals in intergrowth with it in rhyolites from Slavnaya ore zone (Nos. 3, 4—Figures7a and8a) (EPMA data in wt.% and formula).

N Au Ag Cu Pd ∑wt.% NAu Formula Minerals in Intergrowth No.3

p.1a 83.35 12.76 1.92 0.96 98.98 842 Au0.73Ag0.20Cu0.06Pd0.01 Qz; Ab; Kfs; Ath; AuCu; AuV950; X2 p.1a 83.46 13.05 2.21 0.67 99.39 840 Au0.72Ag0.21Cu0.06Pd0.01 -“- p.1c 82.71 7.77 6.46 1.01 97.96 844 Au0.70Ag0.12Cu0.16Pd0.02 Qz; Ab; Kfs; X2 p.1c 84.79 12.9 1.92 0.47 100.08 847 Au0.74Ag0.20Cu0.05Pd0.01 -“- p.1c 84.63 12.56 2.05 1.03 100.28 844 Au0.73Ag0.20Cu0.05Pd0.02 -“- p.1c 89.11 9.17 1.15 1.14 100.56 886 Au0.80Ag0.15Cu0.03Pd0.02 -“- p.1c 84.5 12.37 2.46 0.88 100.21 843 Au0.73Ag0.19Cu0.07Pd0.01 -“- p.3a 83.87 12.46 1.76 0.62 98.71 850 Au0.74Ag0.2Cu0.05Pd0.02 Qz; Ab; Cr-Ms; Ath; AuV930 p.3a 83.93 12.73 1.61 0.57 98.84 849 Au0.73Ag0.21Cu0.05Pd0.02 -“- p.3a 82.19 8.88 7.64 0.41 99.12 829 Au0.67Ag0.13Cu0.19Pd0.01 -“- p.4b 82.42 8.43 7.24 1.2 99.3 830 Au0.67Ag0.13Cu0.18Pd0.02 Qz; Ab; Kfs p.4b 82.6 12.81 1.9 0.98 98.29 840 Au0.73Ag0.21Cu0.05Pd0.02 -“- p.4b 82.03 13.41 1.54 1.11 98.09 836 Au0.73Ag0.21Cu0.05Pd0.02 -“- No.4

p.2 83.64 12.3 2.16 1.14 99.24 843 Au0.74Ag0.2Cu0.06Pd0.02 Qz; Ab; Kfs; Cr-Ms p.2 83.64 11.97 1.41 0.75 97.78 855 Au0.75Ag0.2Cu0.04Pd0.01 -“- p.2 82.57 11.21 3.05 1.2 98.03 842 Au0.73Ag0.19Cu0.06Pd0.02 -“- p.2 84.02 11.18 2.69 1.07 98.96 849 Au0.73Ag0.18Cu0.07Pd0.02 -“- Abbreviations of minerals: native gold (type II, V) (AuII, AuV), fuchsite (Cr-Ms), quartz (Qz), albite (Ab), K-feldspar (Kfs), atheneite (Atn), tetra-aucupride (AuCu) and amoeba phase (X2).

Native gold III is heterogeneous in texture (Figure7h) and contains microveinlets and microinclusions of higher fineness gold (950‰) (Table6) and idiomorphic inclusions of tetra-auricupride (to 10–20 µm in size) of composition Au1.06Cu0.92Pd0.03 (0.7–1.1 wt.% Pd) (Table7). In high-fineness gold, the amount of impurities of Cu increases to 2.3–2.8 wt.% and that of Pd, to 2.2–2.4 wt.%, whereas the content of Ag decreases up to complete disappearance (<0.4 wt.%). The marginal parts of the intergrowth with native gold of type III contain atheneite (Figure7h, p.1a). The atheneite composition Pd 2.62As0.93Hg0.29 differs from the ideal stoichiometric formula Pd2(As0.75Hg0.25) in the higher contents of Pd and As. Native gold of type III is frequently surrounded by thin amoeba-shaped rims (Figure7h , p.1a). The composition of rim substance (phase X2) was not identified. Most likely, this is a mixture of phases (quartz + X2) because, in addition to major elements Si and O, it also contains such elements as Y (3.2), Zr (2.4), Th (1.3), Ce (0.9) (in wt.%) etc. Native gold of type III in association with quartz, albite, and K-feldspar (p.1c) (Figure7d) , (p.3a) (Figure7b) and (p.4b) (Figure7c) of sample No.3 is characterized by a wider range of the contents of elements (in wt.%): from 7.8 to 12.9 Ag and 1.1 to 7.6 Cu, with the same amount of Pd (0.4–1.2), which corresponds to the fineness of 830–890‰ (Table4). The elevated concentrations of Cu (to 6.5* wt.%, Table4) are, most likely, due to the presence of the AuCu phase. In native gold of type III we found microinclusions of higher-fineness gold (930–960‰): the content of Ag decreases down to complete disappearance, Cu is 1.2 wt.%, and Pd increases to 2.8 wt.% (Table6). In sample No.4 (p.2) (Figure8a) naive gold of type III is mainly surrounded by quartz and albite and occasionally has the contact with K-feldspar and fuchsite (Figure8b). Its fineness is 840–850‰, the contents of elements vary (in wt.%): 11.2–12.3 Ag, 1.4–3.1 Cu and 0.8–1.2 Pd (Table4). Minerals 2021, 11, x FOR PEER REVIEW 15 of 26

In sample No.4 (p.2) (Figure 8a) naive gold of type III is mainly surrounded by quartz Minerals 2021, 11, 451 15 of 26 and albite and occasionally has the contact with K‐feldspar and fuchsite (Figure 8b). Its fineness is 840–850‰, the contents of elements vary (in wt.%): 11.2–12.3 Ag, 1.4–3.1 Cu and 0.8–1.2 Pd (Table 4). InIn native native gold gold of of type type III, III, thethe amountamount of of elementselements is is notnot higherhigher thanthan 13.413.4 wt.%wt.% forfor Ag,Ag, 9.69.6 wt.% wt.% Cu Cu and and 1.2 1.2 wt.% wt.% Pd Pd (Table (Table4 ).4). The The elevated elevated contents contents of of Cu Cu in in native native gold gold of of type type IIIIII cancan be be related related to to the the presence presence of of Au-Cu Au‐Cu intermetallide intermetallide in in the the point point probe. probe.

4.2.3.4.2.3. NativeNative GoldGold withwith Ag,Ag, Cu,Cu, Pd,Pd, and and Hg Hg Impurities Impurities NativeNative gold gold of of type type IV IV with with four four elements elements Ag, Ag, Cu, Cu, Pd, Pd, and and Hg Hg was was found found in in rhyolite rhyolite samplesample No.4No. 4 (p.1, (p.1, 5) 5) (Figure (Figures8a,c 8a,c and and Figure 9a). Gold9a). Gold particles particles are in are intergrowth in intergrowth with quartz, with quartz,albite, and albite, K‐ andfeldspar K-feldspar (Figure (Figure 9b, p.1)9b, or p.1) quartz or quartz and fuchsite and fuchsite (Figure (Figure 8c). Figure8c). Figure 9а,f show9a,f showdiscernible discernible lattice latticedecay decaystructures structures with tetra with‐auricupride. tetra-auricupride. The contents The contentsof the following of the followingelements in elements this native in this gold native are (in gold wt.%): are (in 11.9–12.5 wt.%): Ag, 11.9–12.5 1.7–2.5 Ag, Cu, 1.7–2.5 0.6–0.8 Cu, Pd,0.6–0.8 and 0.7–1 Pd, andHg (Table 0.7–1 Hg 5). (Table Its fineness5). Its ﬁnenessvaries in varies the range in the of range 840–870‰. of 840–870 Variations‰. Variations in copper in concen copper‐ concentrationstrations are likely are due likely to duethe presence to the presence of platelets of platelets of AuCu of AuCuphase. phase.In native In gold native of gold type ofIV type there IV are there inclusions are inclusions of U‐mertieite of U-mertieite-II‐II (Figure (Figure9e) and9 e)microinclusions and microinclusions of high of‐fineness high- ﬁnenessgold (type gold V) (type (Figure V) 9a). (Figure 9a).

FigureFigure 9.9. Native gold of of type type IV IV with with decay decay structures structures (matrix—fineness (matrix—ﬁneness 860‰, 860‰ AuCu, AuCu platelets) platelets) and inclusions of higher‐fineness gold (type V) and mertieite (with U impurity) in the quartz‐al‐ and inclusions of higher-ﬁneness gold (type V) and mertieite (with U impurity) in the quartz- bite‐K‐feldspar vein of rhyolite (No. 4, p. 1) (Figure 8a): (a) BSE micrograph; (b–f) maps of areal albite-K-feldspar vein of rhyolite (No.4, p. 1) (Figure8a): ( a) BSE micrograph; (b–f) maps of areal distribution elements (Si, Au, Ag, Pd, Cu) in characteristic rays. Abbreviations of minerals: native distributiongold of type elements IV and V (Si, (AuIV, Au, Ag,AuV), Pd, quartz Cu) in (Qz), characteristic albite (Ab), rays. K‐feldspar Abbreviations (Kfs), U of‐mertieite minerals: (U native‐ goldMert), of tetra type‐ IVauricupride and V (AuIV, (AuCu). AuV), quartz (Qz), albite (Ab), K-feldspar (Kfs), U-mertieite (U-Mert), tetra-auricupride (AuCu).

Minerals 2021, 11, 451 16 of 26

Table 5. Representative analyses of native gold (type IV) and minerals in intergrowth with it in rhyolite from the Slavnaya ore zone (No.4; Figure8a) (EPMA data in wt.% and formula).

N Au Ag Cu Pd Hg ∑wt.% NAu Formula Minerals in Intergrowth

p.1 81.41 11.92 2.32 0.70 0.79 97.43 866 Au0.72Ag0.19Cu0.07Pd0.01Hg0.01 Qz; Ab; Kfs; AuCu; AuV930; U-Mert p.1 82.42 12.02 2.46 0.82 0.96 98.9 865 Au0.72Ag0.19Cu0.07Pd0.01Hg0.01 -“- p.5 81.77 12.2 1.69 0.55 0.76 96.97 843 Au0.74Ag0.2Cu0.04Pd0.01Hg0.01 Qz; Cr-Ms; AuCu p.5 82.23 12.41 1.8 0.52 0.65 97.61 842 Au0.73Ag0.2Cu0.05Pd0.01Hg0.01 -“- p.5 83.03 12.5 1.89 0.72 0.86 99.00 839 Au0.73Ag0.2Cu0.05Pd0.01Hg0.01 -“- Abbreviations of minerals: fuchsite (Cr-Ms), quartz (Qz), albite (Ab), K-feldspar (Kfs), tetra-auricupride (AuCu), U-mertieite (U-Mert).

4.2.4. Native Gold of Fineness 930–1000‰ High-ﬁneness gold (930–1000‰) (type V) is present in all studied rhyolite samples in the form of microveinlets and microinclusions in native gold of types II–IV (see Section 4.2.1, Section 4.2.2 and Section 4.2.3) (Tables2–4). It contains 5.6 wt.% Ag and 2.8 wt.% Cu and Pd (Table6). High-ﬁneness gold with one or two of these impurity elements or pure gold without any impurity also occur (Figure6g,j, Figure7f,h and Figure9a). It is worth noting that Hg is absent in native gold of type V.

Table 6. Representative analyses of high-ﬁneness gold 930–1000‰ (type V) and minerals in intergrowth with it in rhyolites from the Slavnaya ore zone (Nos. 2–4, Figures6a,7a and9a) (EPMA data in wt.% and formula).

N Au Ag Cu Pd ∑wt.% NAu Formula Minerals in Intergrowth No.2

p.6 93.41 1.23 0.93 1.12 96.7 966 Au0.93Ag0.02Cu0.03Pd0.02 AuII860 p.6 89.02 2.14 1.76 1.28 94.2 945 Au0.88Ag0.04Cu0.05Pd0.02 -“- p.6 87.08 2.18 2.27 1.95 93.49 932 Au0.86Ag0.04Cu0.07Pd0 -“- p.7 99.15 0.46 0 0.94 100.55 986 Au0.97Ag0.01Pd0.02 -“- No.3

p.1a 94 0.43 2.34 2.42 99.19 948 Au0.88Ag0.01Cu0.07Pd0.04 AuIII840 p.1a 95.85 0 2.78 2.24 100.87 950 Au0.88Cu0.08Pd0.04 -“- p.1b 97.03 0 1.85 1.08 99.97 971 Au0.93Cu0.05Pd0.02 AuII860 p.1b 97.88 0 0 0 97.88 1000 Au1.0 -“- p.1b 99.04 1.65 0.23 0 100.91 981 Au0.96Ag0.03Cu0.01 -“- p.3a 93.49 5.64 0.55 1.04 100.72 928 Au0.86Ag0.1Cu0.02Pd0.02 AuIII860 p.4b 97.02 0 1.16 2.78 100.96 961 Au0.92Cu0.05Pd0.03 AuII840 No.4

p.1 97.68 0 1.66 1.33 100.17 930 Au0.95Cu0.03Pd0.02 AuIV860 Abbreviations of minerals: native gold (type II, III, IV) (AuII, AuIII, AuIV).

The specific features of high-fineness gold are its high porosity, which resulted in the reduced total contents of components during the analysis (Table6), and recrystallization. These features were observed in sample No.2 (p.6) (Figure6j), in which we detected porous gold of type V with the highest content of Pd (2.8 wt.%) and lower concentrations of Ag and Cu than in the matrix of native gold of types II–IV, which suggests the removal of Ag and Cu and supply of Pd (Table6). High-fineness gold (930–1000 ‰) (type V) in the marginal parts of Ag,Cu-bearing gold (type II) (Figure6j) comes into contact with albite and fuchsite. Minerals 2021, 11, 451 17 of 26

4.2.5. Au-Cu Intermetallides Au-Cu phases were detected in the form of individual microinclusions of idiomorphic shape and thin platelets in lattice decay structures of solid solutions in some large particles of native gold (types II–IV) in rhyolite samples No.2–4. The compositions of Au-Cu intermetallides and matrix for two samples Nos.2, 3 are given in Table7. These data show that Au-Cu intermetallides are similar to AuCu phase with minor amounts of Pd or Ag. The AuCu phase intergrown with native gold of type II and an exsolution texture is absent (Figure7e–h) in sample No.3 (Section 4.2.2). The size of inclusions is no more than 12 µm. The content of Pd is 0.6–1.1 wt.%, which corresponds to the formula Au1.06–1.1Cu0.92–0.88Pd0.02. The AuCu phase in the form of thin platelets (1–2 × 10–20 µm) in lattice decay structures is typical of native gold III in sample No.2 (Figure6a, p.2) (Section 4.2.3) and native gold IV in sample No.4a (Figure9a,f). The content of Pd in the thin platelets in sample No.2 (p.2) does not exceed 0.36 wt.% (Table3), that of Ag varies in a narrow range of 0.3–0.8 wt.%, and the amount of Au is higher than the ideal composition of AuCu (Au1.04–1.06Cu0.92–0.94Pd0.02–0.01Ag0–0.01).

Table 7. Representative analyses of AuCu phases and mineral matrix (EPMA data in wt.% and formula).

N Au Ag Cu Pd ∑wt.% NAu Formula Minerals in Intergrowth No.3

p.1a 76.58 0 21.26 0.75 98.59 777 Au1.06Cu0.92Pd0.02 AuIII840 p.1a 76.35 0 21.4 1.1 98.85 772 Au1.06Cu0.92Pd0.02 -“- p.1b 76.96 0 21.28 0.63 98.87 778 Au1.07Cu0.91Pd0.02 AuIII860 p.1b 77.04 0 21.29 0.73 99.06 778 Au1.07Cu0.91Pd0.02 -“- p.1b 77.01 0 19.80 0.62 97.43 790 Au1.1Cu0.88Pd0.02 -“ - No.2

p.2 77.20 0.29 22.37 0.36 100.23 770 Au1.04Cu0.94Pd0.01Ag0.01 AuII830–860 p.2 77.12 0.84 21.52 0 99.48 775 Au1.06Cu0.92Ag0.02 -“- Abbreviations: native gold of types II and III (AuII, AuIII).

We failed to determine the composition of the Au-Cu phases in native gold of type IV in sample No.4a (Figure9a,f) owing to poor visualization on the optical and scanning microscopes and small thickness of the plates (<1 µm).

4.3. Reconnaissance of Fluid Inclusions A limited number of fluid inclusions were studied in albite and quartz in rhyolites from the Slavnaya and Ludnaya ore zones (Figure 10). While the data are too few data to draw firm conclusions, they are sufficient to allow us to hypothesize about the nature of the hydrothermal system. In rhyolite samples from the Slavnaya zone, albite and quartz are associated with native gold of types II–IV, which suggests their synchronous crystallization (Figures6c,e and7a–e). In rhyolite samples from the Ludnaya zone, albite and quartz do not contain gold particles, but are intergrown with fuchsite in which native gold of type I is localized (Figures3 and4). Fluid inclusions are classified as primary inclusions when they occur along growth zones (Figure 10a,b) or are isolated (Figure 10c,d). Secondary inclusions in quartz form trails in cracks. The size of fluid inclusions is no more than 10–15 µm. Minerals 2021, 11, 451 18 of 26 Minerals 2021, 11, x FOR PEER REVIEW 18 of 26

Figure 10. Albite crystals saturated with primary fluid inclusions (a); primary vapor‐liquid two‐ Figure 10. Albite crystals saturated with primary ﬂuid inclusions (a); primary vapor-liquid two- phase and liquid single‐phase fluid inclusions tracing the growth zone in albite (b) (a,b)—Slavnaya phase and liquid single-phase ﬂuid inclusions tracing the growth zone in albite (b)(a,b)—Slavnaya ore zone); primary two‐phase and vapor inclusions in quartz (c); primary vapor inclusions in quartz ore(d) zone); (c,d)—Ludnaya primary two-phase ore zone). and vapor inclusions in quartz (c); primary vapor inclusions in quartz (d)(c,d)—Ludnaya ore zone).

4.3.1.4.3.1. The The Ludnaya Ludnaya Ore Ore Zone Zone OnlyOnly 9 9 primary primary ﬂuid fluid inclusions inclusions were were analyzed analyzed from from the the Ludnaya Ludnaya ore ore zone. zone. In In albite albite andand quartz, quartz, two-phasetwo‐phase inclusions are are homogenized homogenized in in the the temperature temperature range range of of186 186 to 139 to 139°С ◦(TableC (Table 8).8 Melting). Melting of ofeutectic eutectic takes takes place place at at −39− °39С.◦ TheC. The temperature temperature is close is close to the to the eu‐ tectic temperature of the MgCl2‐KCl‐H2O system (−37.8 °С◦). Complete melting of ice in eutectic temperature of the MgCl2-KCl-H2O system (−37.8 C). Complete melting of ice infrozen frozen inclusions inclusions in in albite albite takes takes place place at − at0.3− 0.3to 0.1 to 0.1°С. ◦TheC. The salinity salinity of solutions of solutions varies varies from from2.1 to 2.1 0.2 to wt.% 0.2 wt.% NaCl NaCl eq. eq.

TableTable 8. 8.Results Results of of primary primary two-phase two‐phase ﬂuid fluid inclusion inclusion study study by by cryo- cryo and‐ and thermometry. thermometry.

◦ ◦ ◦ Salinity (wt.% SampleSample Mineral Mineral Th °CС TeuTeuC °С Tice TiceC °С Salinity (wt.% NaCl eq.) NaCl eq.) Rhyolite from Ludnaya ore zone Rhyolite from Ludnaya ore zone 3p-1154–2 (4) albite 155–139 −39 −0.3 to −0.1 0.5–0.2 3р‐3p-1154–21154–2 (5) quartz 186–178 −39 −1.2 to −0.7 2.1–1.2 albite 155–139 −39 −0.3 to −0.1 0.5–0.2 (4) Rhyolite from Slavnaya ore zone 3р‐1154–2 − − − − 3p-1122-1 (10) *quartz albite 186–178 130–113 −39 to 3938 −15 to1.27.5 to −0.7 18.6–112.1–1.2 3p-1122-1(5) (5) quartz 165–105 −55 −10 to −7.5 13.9–11 3p-1122-13 (4) albite 133–125 −49 −17 to −13 20.2–16.9 Rhyolite from Slavnaya ore zone Note: * —number of fluid inclusions; Th—homogenization temperature, Teu—melting of eutectic and Tice— melting3p‐1122 of ice.‐1 albite 130–113 −39 to −38 −15 to −7.5 18.6–11 (10) * 4.3.2.3p‐1122 The‐1 Slavnaya (5) quartz Ore Zone 165–105 −55 −10 to −7.5 13.9–11 3р‐Only1122‐ 1913 primary fluid inclusions were analyzed from the Slavnaya ore zone. In quartz albite 133–125 −49 −17 to −13 20.2–16.9 and albite,(4) two-phase fluid inclusions are homogenized in the temperature range from 165 ◦ ◦ toNote: 105 * C.—number In these of inclusions, fluid inclusions; melting Th—homogenization of eutectic is observed temperature, at −49 Teu—meltingC, which isof similareutectic ◦ toand the Tice—melting melting temperatures of ice. of eutectic of the CaCl2-KCl-H2O(−50.5 C) and CaCl2-H2O (−49.8 ◦C) systems. Ice melts at temperatures from −15 to −7.5 ◦C. The salinity of solutions of4.3.2. fluid The inclusions Slavnaya varies Ore Zone from 20.2 to 11 wt.% NaCl eq. (Table8). Only 19 primary fluid inclusions were analyzed from the Slavnaya ore zone. In quartz and albite, two‐phase fluid inclusions are homogenized in the temperature range from 165 to 105 °С. In these inclusions, melting of eutectic is observed at −49 °С, which is

Minerals 2021, 11, 451 19 of 26

5. Discussion The mechanism of formation of native gold of varying chemical composition is very complicated and depends on many factors [22–28]. The content of elements in native gold is determined by their concentrations in hydrothermal solutions, which are governed by temperature, redox conditions, pH, presence of ligand elements Cl, S, and elements such as Se, Te, As, Sb. A wide spectrum of impurity elements in native gold is related to different geochemical conditions in which mobilization, transportation, and deposition of gold-ore mineralization took place. Nowadays, several hypotheses on the role and ratios of magmatic, metamorphic, and hydrothermal processes during the formation of Au-Pd mineralization are discussed. Fluids are considered to play an important role in the genesis of mafic-ultramafic complexes with mineralization of noble metals [29–36]. The efficiency of mobilization, transportation, and deposition of noble metals with participation of fluids at the magmatic and post-magmatic stages of evolution of the ore-forming process is shown in the experiments [37]. The precipitation of these metals from hydrothermal solutions is the main process of concentrating of metals and they can form their own mineral phases or occur as an isomorphic impurity in other minerals. The fineness and set of impurity elements in native gold, and associated minerals are the indicators of different genetic origin [28,38–43]. Pd-bearing native gold occurs both in magmatic sulfide ores and in low-sulfide post-magmatic metasomatites [44,45]. Large occurrences of low-sulfide, essentially Au- Pd mineralization in different types of mafic-ultramafic complexes are known in the Norilsk district (Talnakhskoye, Norilsk-1 and 2, Chernogorskoe, Vologochanskoe), in the Skaergaard formation, in John Melville reef of the Stillwater complex and some others [46]. The elevated Cu content in native gold indicates the probable genetic relation of gold mineralization with mafic-ultramafic complexes or with the deposits of copper type (copper- skarn, copper-pyrite and porphyry copper) [47–50].

5.1. Compositions of Native Gold and Minerals in Intergrowth The study reveals differences in the set and quantity of impurity elements in the composition of native gold, intergrown minerals, and PTX parameters of the deposition of Au-Pd mineralization in two ore zones of the Chudnoe deposit. The obtained results (Figure 11a) together with the earlier published data (Figure 11b) allowed us to distinguish five types of native gold. Both the data from previous works [2,3,5] and our results (Table2) show that native gold (type I) from the Ludnaya ore zone contains only Ag. The fineness of native gold varies within the range of 510 to 790‰. Native gold in this ore zone occurs in the form of impregnated particles commonly intergrown with arsenoantimonide in a fuchsite or allanite matrix (Figures2–5). Palladium arsenoantimonide intergrown with native gold (type I) is represented by U- or Cu-mertieite-II. In this type of native gold, we did not detect Pd, Cu, and Hg impurities even under long-term accumulation of spectra (see Section3). In rhyolites of the Slavnaya ore zone, native gold is heterogeneous, has a higher fineness, different sets and amounts of impurities: II type—Au-Ag-Cu solid solution (840–860‰); III—Au-Ag-Cu-Pd (830–890‰), IV—Au-Ag-Pd-Cu-Hg (840–870‰), as well as a specific mineral composition of the surrounding matrix-fuchsite or allanite, albite, and mertieite-II (II); albite, quartz, and atheneite (III); quartz, albite, K-feldspar, and mertieite-II (IV) (Tables3–5, Figures3–9). It is worth noting that native gold of II and IV types shows some similarity with native gold of type I, as it occurs in paragenesis with fuchsite or allanite (II type) and mertieite-II (types II and IV). Minerals 2021, 11, x FOR PEER REVIEW 20 of 26 Minerals 2021, 11, 451 20 of 26

Figure 11.11. Chemical composition of various types of native gold from the Chudnoe deposit deposit on on the the ternaryternary diagramdiagram Au-Ag-Cu(+ Au‐Ag‐Cu(+ Pd Pd + + Hg) Hg) from from the the results results of of present present study study (a )(а and) and data data of otherof other authors (authorsb): 1—[ 1(b];): 2—[ 1—[1];2]; 3—[ 2—[2];5]; 4—[ 3—[5];3]; 5 4—[3]; [7,14–16 5 ].[7,14–16].

InIn thisrhyolites study of we the revealed Slavnaya lattice ore decayzone, structuresnative gold with is heterogeneous, tetra-auricupride has AuCu a higher for nativefineness, gold different of types sets III (Ag,Cu,Pd) and amounts and of IV impurities: (Ag,Cu,Pd,Hg). II type—Au AuCu‐Ag intermetallide‐Cu solid solution is present (840– in native860‰); gold III—Au of type‐Ag II‐Cu but‐Pd in the(830–890‰), form of isometric IV—Au microinclusions.‐Ag‐Pd‐Cu‐Hg (840–870‰), Intermetallides as ofwell AuCu, as a Auspecific3Cu, Aumineral3Cu2, Aucomposition2Cu, AuCu of3 compositionthe surrounding were reportedmatrix‐fuchsite by other or authors allanite, [5 ,8albite,,16,50 ,and51]. mertieiteThe‐ phaseII (II); diagramalbite, quartz, Au-Cu and [52 atheneite] shows (III); that thequartz, presence albite, of K‐ latticefeldspar, decay and structures mertieite‐ withII (IV) thin (Tables platelets 3–5, Figures of tetra-aucupride 3–9). It is worth AuCu noting in native that goldnative indicates gold of II temperatures and IV types410 shows◦C. Au-Cusome similarity intermetallides with native of composition gold of type Au I,3Cu as andit occurs AuCu in3 paragenesisare formed atwith temperatures fuchsite or belowallanite 390 (II and type) 240 and◦C. mertieite The formation‐II (types of decay II and structures IV). in the initially homogeneous solid solutionIn this results study from we the revealed decrease lattice in the decay miscibility structures of components with tetra and‐auricupride their redistribution AuCu for withnative decreasing gold of types in temperature. III (Ag,Cu,Pd) Such and structures IV (Ag,Cu,Pd,Hg). were also found AuCu at intermetallide other objects: is Zolotaya present Gora,in native Melent’evskoe gold of type deposits II but in (Southern the form Urals, of isometric Russia) microinclusions. [53,54], Agardag Intermetallides ultramafic massif of (S.AuCu, Tuva, Au Russia)3Cu, Au [443],Cu the2, Au 15 Mile2Cu, depositAuCu3 (thecomposition Dease Lake were district, reported British by Columbia) other authors [55], Au-Pd[5,8,16,50,51]. ores of the Skaergaard massif (Greenland) [45]. The phase diagram Au‐Cu [52] shows that the presence of lattice decay structures 5.2.with Inferences thin platelets on the of PTX tetra Parameters‐aucupride of AuCu Au-Pd-REE in native Mineralization gold indicates temperatures 410 °С. Au‐CuData intermetallides on the PTX parameters of composition of formation Au3Cu, of Au-Pd-REE Au3Cu2, Au mineralization2Cu, AuCu3 atare the formed Chudnoe at deposittemperatures were forbelow the first390 timeand 240 obtained °С. The by Surenkovformation etof al. decay [12,56 structures]. They used in the thermocry- initially ometryhomogeneous to analyze solid 110 solution vapor-liquid results from inclusions the decrease of which in thethe vapor-liquidmiscibility of inclusions components in albiteand their and redistribution pre-ore quartz with were decreasing attributed in to temperature. primary inclusions. Such structures In early were veined also quartz, found ◦ homogenizationat other objects: Zolotaya temperature Gora, ranged Melent’evskoe from 230 to deposits 400 C, and(Southern salt concentrations, Urals, Russia) from [53,54], 2.1 toAgardag 17 wt.% ultramafic NaCl eq. massif For late (S. generations Tuva, Russia) of quartz, [44], the albite 15 Mile and calcite,deposit which (the Dease reflect Lake the conditionsdistrict, British for the Columbia) formation [55], of Au Au-Pd‐Pd ores mineralization, of the Skaergaard temperature massif (Greenland) ranged from [45]. 100 to 180 ◦C and the concentrations of salts, from 2.5 to 23 wt.% NaCl eq. Surenkov [12] suggests that5.2. Inferences at the early on stage, the PTX the Parameters Chudnoe deposit of Au‐Pd was‐REE formed Mineralization with participation of metamorphic fluids and the formation of Au-Pd mineralization took place at lower temperatures with Data on the РТХ parameters of formation of Au‐Pd‐REE mineralization at the participation of meteoric, marine, or buried waters. Chudnoe deposit were for the first time obtained by Surenkov et al. [12,56]. They used Our limited study of fluid inclusions are generally consistent with the data of thermocryometry to analyze 110 vapor‐liquid inclusions of which the vapor‐liquid Surenkov et al. [12,56]. Ore-forming fluids of Ludnaya and Slavnaya ore zones range inclusions in albite and pre‐ore quartz were attributed to primary inclusions. In early salinity from 2.1 to 0.2 and from 20.2 to 11 wt.% NaCl eq., respectively. It is known, how- veined quartz, homogenization temperature ranged from 230 to 400 °С, and salt ever, that mineral parageneses with quartz, albite and sericite (fuchsite) in gold deposits are concentrations, from 2.1 to 17 wt.% NaCl eq. For late generations of quartz, albite and formed at the temperatures higher than 200–230 ◦C[57]. At lower temperatures, argillisites form,calcite, in whichwhich sericitereflect is the commonly conditions replaced for bythe mixed-layer formation minerals of Au‐ ofPd clay mineralization, series. These datatemperature allow us ranged to estimate from the 100 possible to 180 ° pressureС and the of concentrations ore formation by of comparingsalts, from the2.5 t formationо 23 wt.% temperatureNaCl eq. Surenkov of the [12] quartz-albite-sericite suggests that at the (fuchsite) early stage, mineral the Chudnoe association deposit in gold was deposits formed with isochores of fluid inclusions solutions.

Minerals 2021, 11, x FOR PEER REVIEW 21 of 26

with participation of metamorphic fluids and the formation of Au‐Pd mineralization took place at lower temperatures with participation of meteoric, marine, or buried waters. Our limited study of fluid inclusions are generally consistent with the data of Surenkov et al. [12,56]. Ore‐forming fluids of Ludnaya and Slavnaya ore zones range salinity from 2.1 to 0.2 and from 20.2 to 11 wt.% NaCl eq., respectively. It is known, however, that mineral parageneses with quartz, albite and sericite (fuchsite) in gold deposits are formed at the temperatures higher than 200–230 °С [57]. At lower temperatures, argillisites form, in which sericite is commonly replaced by mixed‐layer Minerals 2021, 11, 451 minerals of clay series. These data allow us to estimate the possible pressure 21of of ore 26 formation by comparing the formation temperature of the quartz‐albite‐sericite (fuchsite) mineral association in gold deposits with isochores of fluid inclusions solutions. Gold mineralization at the Ludnaya ore zone was formed synchronously with the Gold mineralization at the Ludnaya ore zone was formed synchronously with the quartz‐albite‐fuchsite association. Thus, the trapping pressure of fluid inclusions could quartz-albite-fuchsite association. Thus, the trapping pressure of ﬂuid inclusions could vary from 23 to 114 MPa at temperatures from 200 to 230 °C (Figure 12) in this zone. Gold vary from 23 to 114 MPa at temperatures from 200 to 230 ◦C (Figure 12) in this zone. Gold mineralization at the Slavnaya ore zone was formed later than the quartz‐albite‐fuchsite mineralization at the Slavnaya ore zone was formed later than the quartz-albite-fuchsite association at 105–165 °С, the trapping pressure of fluid inclusions could vary from 5 to association at 105–165 ◦C, the trapping pressure of ﬂuid inclusions could vary from 5 to 115115 MPaMPa (Figure(Figure 1212).).

Figure 12. Isochores of fluidsfluids fromfrom thethe SlavnayaSlavnaya zonezone (solid(solid line)line) andand fluidsfluids fromfrom thethe LudnayaLudnaya zonezone (dashed(dashed line).line). IsochoresIsochores werewere constructedconstructed usingusing thethe “AqSo-NaCl:“AqSo‐NaCl: computercomputer program”program” forfor extremeextreme temperatures and homogenization concentrations [21]. The probable trapping PT conditions of fluid temperatures and homogenization concentrations [21]. The probable trapping PT conditions of fluid inclusions are highlighted in light gray for the Slavnaya zone (I) and in dark gray for Ludnaya zone inclusions are highlighted in light gray for the Slavnaya zone (I) and in dark gray for Ludnaya (II). zone (II).

Thus, it can be assumed that the PT parameters of formation of Au-PdAu‐Pd mineralization associated withwith fuchsite, fuchsite, especially especially in early in early rhyolites rhyolites from the from Slavnaya the Slavnaya zone, are suggestedzone, are tosuggested be higher to than be higher those estimated than those in theestimated fluid inclusion in the fluid study. inclusion The fluids study. forming The Au-Pd fluids mineralizationforming Au‐Pd at mineralization the Chudnoe depositat the Chudnoe are similar deposit in salt are composition similar in salt to the composition ore-forming to fluidsthe ore of‐forming hydrothermal fluids Au-Ag of hydrothermal deposits, though Au‐Ag the latterdeposits, are characterizedthough the bylatter higher are formationcharacterized temperatures, by higher formation higher salinity temperatures, of fluids andhigher frequent salinity presence of fluids of and dense frequent gases presence of dense gases (CO2, N2, CH4) in the composition of fluids [58]. They are similar (CO2,N2, CH4) in the composition of fluids [58]. They are similar in temperature, concentration,in temperature, and composition concentration, of salts and (Ca-Na-K composition chlorides, of salts carbonates, (Ca‐Na‐K and chlorides, hydrocarbonates) carbonates, toand the hydrocarbonates) ore-forming fluids to the of Au-Pd ore‐forming deposits fluids (Serra of Au Pelada,‐Pd deposits Bleida Far(Serra West), Pelada, for whichBleida anFar infiltration West), for modelwhich ofan formation infiltration with model participation of formation of oxidized with participation basin Na-Ca of chlorideoxidized watersbasin isNa assumed‐Ca chloride [59–61 ].waters However, is assumed these deposits [59–61]. are characterizedHowever, these by the deposits presence are of Se-bearingcharacterized PGM by (Pd–Pt–Se,the presence Pd–Se, of Se‐ Pd–Hg–Se,bearing PGM and (Pd–Pt–Se, Pd–Bi–Se Pd–Se, phases, Pd–Hg–Se, and sudovikovite and Pd– andBi–Se palladseite) phases, and [62 sudovikovite]. and palladseite) [62]. Seven metals (Ag, Cu, Pd, Hg, Sn, Tl, Fe), three chalcogenes (Te, S,S, Se)Se) andand threethree metalloids (As, (As, Sb, Sb, Bi) Bi) can can be be indicators indicators of ofthe the presence presence of gold of gold minerals minerals in ores in [63]. ores Four [63]. Four of these metals—Ag, Cu, Pd, and Hg—were detected in the composition of native gold from the Slavnaya ore zone (Chudnoe deposit). Cu also formed intermetallides with Au. As, Sb, and Hg, found in elevated concentrations in the Chudnoe ores, under specific conditions of ore formation were deposited as Pd minerals with these elements—arsenides (atheneite), arsenoantimonides (mertieite-I, mertieite-II, isomertieite), antimonides (stibiopalladinite) [1–3,8]. Zaccarini et al. [64] think that the Pd content in native gold is determined by the evolution of S, Te, As, Sb, Bi, Se concentrations in a fluid, which bind palladium into its own minerals. Many authors [4,64,65] suppose that with a decrease in temperature and change in redox conditions to more oxidizing, the concentrations of sulfide sulfur decrease and those of the palladium binding elements increase. Yanakieva and Spiridonov [36,66] reported that Ag-Au-Pd minerals are typical of telethermal gold deposits formed at low Minerals 2021, 11, 451 22 of 26

f S2 and elevated f O2. They are regarded as the result of deposition of ore components from chloride hydrotherms that have a high oxidation potential. As selenides, tellurides, and bismuthides are not typical of the Chudnoe deposit, the composition of native gold is likely determined by the evolution of Au, Ag, Cu, Pd, Hg, As, Sb concentrations in a ﬂuid, which bind palladium into its own minerals—sulfoantimonides, atheneite, etc. The data of Borisov from [5] show that the formation of Au-Pd-REE mineralization of ore occurrences is the latest hydrothermal event related to the regressive stage of the Late Hercynian metamorphism. The formation of Au-Pd-REE mineralization of the Chudnoe deposit resulted from the oxidation of ascending reduced metamorphic solutions, which was also accompanied by an increase in their acidity. The part of the oxidation geochemical parameter was played by hematite-rich rocks in the zone of regional unconformity. The conditions of ore deposition at the Chudnoe deposit, according to this author, correspond ◦ to log f O2 ~ −47, pH ~ 4.5 at 150 C. Changes in the set of minerals and their composition are due to the vertical variability of redox conditions and acidity of solutions, which in turn are a result of the different localizations of objects relative to the regional unconformity.

5.3. Genesis of Au-Pd Mineralization, Sources of Ore Components The genesis of the Chudnoe deposit is still debatable. There are several viewpoints on the origin of complex mineralization of this uncommon type. Tarbaev and coauthors [1] adhered to the hydrothermal genesis of Au-Pd-REE ores at the Chudnoe deposit, while Cr, Pd, Au, and Cu were mobilized from presumably deep-seated mafic and ultramafic rocks, probably, andesite-basalts from the lower series of the Upper Riphean Sablegorskaya formation, and K and lanthanides, from the host porphyry rhyolites. Some researchers of the Chudnoe deposit [5,12,13] suggest a metamorphic-hydrothermal model of formation: ascending metamorphic hydrothermal solutions mobilized metals from underlying rocks and the main factors of ore formation are oxidation and increase in the acidity of ore- bearing solutions. Originally, metamorphic solutions were reduced (field of stability of pyrite and magnetite), weakly-acidic—near-neutral (acidity was controlled by the quartz- K-feldspar-muscovite association). At the later stages of the process, during the formation of Au-Pd mineralization, considerable amounts of meteoric and buried waters participated in the hydrothermal system [12]. On the basis of the results of isotopic and mineralogical study, Galankina [2] shows that Au-Pd-REE mineralization of the Chudnoe deposit should be regarded as hydrothermal, originated in the Riphean after the formation of Sablegor and Maldin rhyolites, occurring in the zone of the deep Maldinskiy fault and represented by pre-ore metasomatites which are the most similar to the berezite-listvenite formation and ore-accompanying near-crack quartz-albite metasomatites and listvenite-like rocks. There are reasons to assume that Cr and Pd mineralization of the Chudnoe deposit is related to the fact that hydrothermal processing of riftogenic complexes localized in the Maldinskiy fault involved ultramafic rocks that are not exposed on the surface. Such complexes of the Riphean age widely occur within the Central-Ural uplift, e.g., Saranovskii massif. Magnetic survey data show the occurrence of a positive magnetic anomaly at the depth, which suggests that the intrusion chamber could be the source of the ore-forming fluid [67]. Co-occurrence of native gold and fuchsite was observed at many gold deposits [68–70]. The associations of fuchsite with native gold can be found in altered ultramafic rocks at Kalgoorlie (Australia), the Kerr-Addison and Dome mines, in the Virginiatown and Porcupine mining districts of northeast Ontario, Canada; the Mother Lode district in California, USA; in the Transvaal district of South Africa (including, confusingly, the Murchison Range); British Columbia and the Yukon, Quebec and Newfoundland; Ireland; Morocco; Egypt; and Saudi Arabia [68]. However, these deposits are characterized by a listvenite or QAM (quartz-ankerite-mariposite) type of hydrothermal alteration formed by the mafic-ultramafic host rocks. Native gold in listvenites is also typical of many Ural objects [71]. Mechnikovskoe, Altyn-Tash, and Ganeevskoe belong to the listvenite-related gold deposits, which occur Minerals 2021, 11, 451 23 of 26

in the large Main Uralian fault zone and some smaller faults within the Magnitogorsk zone (South Urals). Listvenites are developed after serpentinites and composed of quartz, fuchsite (or mariposite), and carbonates (magnesite, dolomite) ± albite. Volcanic and volcanoclastic rocks are altered to beresites, consisting of sericite, carbonates (dolomite, ankerite), quartz, and albite. The process of alteration occurs under the influence of CO2- and S-rich fluids [72–74]. At the Chudnoe deposit under study, native gold is associated with fuchsite, but in the absence of carbonates and sulfides, which makes it different from other objects with listvenite or QAM (quartz-ankerite-mariposite) type of hydrothermal alteration and suggests the participation of CO2-free or low-CO2 fluid. The formation of native gold I (only with Ag impurity) in rhyolites of the Ludnaya ore zone is, most likely related to the processes of fuchsitization and allanitization of rhyolites with the supply of Cr and REE. The formation of native gold with Ag, Cu, Pd, and Hg impurities (types II–IV), supposedly, took place during the following metasomatic processes: silicification, albitization, and feldspathization in rhyolites. The intersections of quartz-albite metasomatites with fuchsite veinlets support such an assumption. The porous texture of high-fineness gold (type V) is explained by the removal of Cu and Ag. The association of native gold with palladium minerals and presence of Cu and Pd in it, Cr in fuchsite indicate the relationship between ore formation and mafic-ultramafic magmatism.

Author Contributions: Conceptualization, methodology, G.P. and V.M.; investigation, G.P., N.K., and A.B.; writing and editing, G.P., V.M., N.K. and A.B.; visualization G.P., V.M., S.K. and A.B.; supervision G.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded with financial support from the Russian Foundation of Basic Research (project No 20-05-00393) and by within the framework of the state assignment of Sobolev Institute of Geology and Mineralogy, Zavaritsky Institute of Geology and Geochemistry (AAAA-A18- 118052590028-9), Institute of Geology, Komi Science Center (Russian Academy of Sciences). Acknowledgments: We are very grateful to the Reviewers and Academic Editors for their helpful comments that improved our manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Tarbaev, M.B.; Kuznetsov, S.K.; Moralev, G.V.; Soboleva, A.A.; Laputina, I.P. New gold-palladium type of mineralization in the Kozhim Region of Circumpolar Ural (Russia). Geol. Ore Depos. 1996, 38, 11–25. 2. Galankina, O.L.; Gavrilenko, V.V.; Gaydamako, I.M. New data on mineralogy of hydrothermal gold-platinoid mineralization of the Subpolar Urals. Zapiski RMO 1998, 3, 72–78. (In Russian with English abstract) 3. Shumilov, I.K.; Ostashchenko, B.A. Mineralogical and Technological Peculiarities of Au-Pd-TR Metallization in the Subpolar Urals; Geoprint: Syktyvkar, Russia, 2000; p. 104. 4. Anikina, E.V.; Alekseev, A.V. Mineral-geochemical characteristic of gold-palladium mineralization in the Volkov gabbro massif (Platiniferous Urals Belt). Litosfera 2010, 5, 75–100. (In Russian with English abstract) 5. Borisov, A.V. Geological and Genetic Features of Au-Pd-REE Ore Occurrences in the Maldy-Nyrd Ridge (Subpolar Urals). Ph.D. Thesis, IGEM RAS, Moscow, Russia, 2005; p. 27. (In Russian) 6. Nikulova, N.Y.; Filippov, V.N. Native palladium in gold from conglomerates of the Telpos (O1TP) Formation Muldynyrd Ridge, Subpolar Urals. J. Proc. Komi Sci. Centre Ural Branch RAS 2009, 138, 69. (In Russian with English abstract) 7. Onishchenko, S.A.; Kuznetsov, S.K. Palladium-gold sulﬁde mineralization in andesites at the Chudnoe deposit (Subpolar Urals). J. Proc. Komi Sci. Centre Ural Branch RAS 2019, 294, 20–27. (In Russian with English abstract) 8. Kuznetsov, S.K.; Mayorova, T.P.; Shaibekov, R.I.; Sokerina, N.V.; Filippov, V.N. Mineral composition and conditions of formation of gold-platinum-palladium occurrences in the north of the Urals and Pai-Khoi. Geol. Miner. Resour. 2014, 3, 81–85. (In Russian with English abstract) 9. Pystin, A.M.; Potapov, I.L.; Pystina, Y.I.; Generalov, V.I.; Onishchenko, S.A.; Filippov, V.N.; Shloma, A.A.; Tereshko, V.V. Low-Sulﬁde Platinum-Metal Mineralization in the Polar Urals; Pystin, A.M., Ed.; Institute of Economics Ural Branch of RAS: Yekaterinburg, Russia, 2011; p. 150. (In Russian with English abstract) 10. Ozerov, V.S. Metamorfogennye rossypi zolota Pripolyarnogo Urala (Metamorphogenic gold placers in the Subpolar Urals). Ores Metals 1996, 4, 28–37. (In Russian) 11. Galankina, O.L. Peculiarities of Mineralogy of Palladium-Gold Occurrences of the Subpolar Urals. Ph.D. Thesis, St. PMU, Saint Petersburg, Russia, 2001; p. 156. (In Russian) Minerals 2021, 11, 451 24 of 26

12. Surenkov, S.V. Formation Conditions and Sources of Ore Matter Au-PGE-REE of Ore Occurrences in Alkesvozhskaya Area (Subpolar Urals). Ph.D. Thesis, IGEM RAS, Moscow, Russia, 2003; p. 23. (In Russian) 13. Moralev, G.V.; Borisov, A.V.; Surenkov, S.V.; Tarbaev, M.B.; Ponomarchuk, V.A. First 39Ar-40Ar datings on micas from the Chudnoe Au-Pd-REE occurrence, Near-Polar Urals. Dokl. Earth Sci. 2005, 400, 109–112. 14. Kuznetsov, S.K.; Onishchenko, S.A. Gold content of local areas of metasomatic alteration of rhyolites of the Chudnoe deposit (Subpolar Urals). J. Proc. Komi Sci. Centre Ural Branch RAS 2018, 288, 39–45. (In Russian with English abstract) 15. Kuznetsov, S.; Mayorova, T.; Sokerina, N.; Glukhov, Y. Gold-bearing areas of the western slope of Northern Urals and Timan. J. Proc. Komi Sci. Centre Ural Branch RAS 2018, 4, 81–94. [CrossRef] 16. Onishchenko, S.A.; Kuznetsov, S.K.; Tropnikov, E.M. Epigenetic alteration of cupreous gold in the Au-Ag-Cu-Pd exsolution texture. Dokl. Earth Sci. 2020, 492, 418–421. [CrossRef] 17. Lafuente, B.; Downs, R.T.; Yang, H.; Stone, N. The power of databases: The RRUFF project. In Highlights in Mineralogical Crystallography; Armbruster, T., Danisi, R.M., Eds.; De Gruyter: Berlin, Germany, 2015; pp. 1–30. 18. Chernova, A.E.; Moralev, G.V.; Tarbaev, M.B.; Kuznetsov, S.K.; Wolfson, A.A. Distribution and forms of occurrence of mercury in the Au-Pd-REE ore occurrence Chudnoye, Kozhimsky region, Subpolar Urals. In The Main Problems of Teaching About Magmatoid Ore Deposits; IGEM RAS: Moscow, Russia, 1997; pp. 172–173. (In Russian) 19. Borisenko, A.S. Cryometric analysis of salt composition of solutions of gas-liquid inclusions in minerals. In Application of Therobarogeochemical Methods in Prospecting and Study of Ore Deposits; Nedra: Moscow, Russia, 1982; pp. 37–47. (In Russian) 20. Bodnar, R.J.; Vityk, M.O. Interpretation of microthermometric data for NaCl-H2O fluid inclusions, Fluid Inclusions in Minerals. In Methods and Applications; De Vivo, B., Frezzotti, M.L., Eds.; Virginia Polytechnic Institute State University: Blacksburg, VA, USA, 1994; pp. 117–131. 21. Bakker, R.J. AqSo NaCl: Computer program to calculate p-T-V-x properties in the H2O-NaCl fluid system applied to fluid inclusion research and pore fluid calculation. Comput. Geosci. 2018, 115, 122–133. [CrossRef] 22. Fisher, N.H. The fineness of gold, with special reference to the Morobe gold field, New Guinea. Econ. Geol. 1945, 40, 449–495. [CrossRef] 23. Morrison, G.W.; Rose, W.J.; Jaireth, S. Geological and geochemical controls on the silver content (fineness) of gold in gold-silver deposits. Ore Geol. Rev. 1991, 6, 333–364. [CrossRef] 24. Gammons, C.H.; Williams-Jones, A.E. Hydrothermal geochemistry of electrum; thermodynamic constraints. Econ. Geol. 1995, 90, 420–432. [CrossRef] 25. Pal’yanova, G. Physicochemical modeling of the coupled behavior of gold and silver in hydrothermal processes: Gold fineness, Au/Ag ratios and their possible implications. Chem. Geol. 2008, 255, 399–413. [CrossRef] 26. Liang, Y.; Hoshino, K. Thermodynamic calculations of AuxAg1−x-Fluid equilibria and their applications for ore-forming conditions. Appl. Geochem. 2015, 52, 109–117. [CrossRef] 27. Palyanova, G.; Zinina, V.; Kokh, K.; Seryotkin, Y.; Zhuravkova, T.; Mikhlin, Y. New gold chalcogenides in the Au-Te-Se-S System. J. Phys. Chem. Solids 2020, 138, 109276. [CrossRef] 28. Chapman, R.J.; Banks, D.A.; Styles, M.T.; Walshaw, R.D.; Piazolo, S.; Morgan, D.J.; Grimshaw, M.R.; Spence-Jones, C.P.; Matthews, T.J.; Borovinskaya, O. Chemical and physical heterogeneity within native gold: Implications for the design of gold particle studies. Miner. Deposita 2021, 1–26. [CrossRef] 29. Letnikov, F.A. Fluid regime of endogenous processes and problems of ore genesis. Rus. Geol. Geophys. 2001, 47, 1296. 30. Williams-Jones, A.E.; Bowell, R.J.; Migdisov, A.A. Gold in Solution. Elements 2009, 5, 281–287. [CrossRef] 31. Murzin, V.V.; Sazonov, V.N. Origin of cupriferous gold mineralization in Alpine-type ultramafic rocks. Dokl. Earth Sci. 1999, 367, 634–635. 32. Duuring, P.; Hagemann, S.G.; Cassidy, K.F.; Johnson, C.A. Hydrothermal Alteration, Ore Fluid Characteristics, and Gold Depositional Processes along a Trondhjemite-Komatiite Contact at Tarmoola, Western Australia. Econ. Geol. 2004, 99, 423–451. [CrossRef] 33. Rauchenstein-Martinek, K.; Wagner, T.; Walle, M.; Heinrich, C.A. Gold concentrations in metamorphic fuids: A LA-ICPMS study. Chem. Geol. 2014, 385, 70–83. [CrossRef] 34. Prokof’ev, V.Y. Main Principles of Hydrothermal Deposit Typification Based on Fluid Inclusion Study: The Case of Gold. Trans. Rus. Acad. Sci. Earth Sci. Sect. 2003, 354, 2553–2555. (In Russian) 35. Mountain, B.W.; Wood, S.A. Chemical controls on the solubility, transport and deposition of platinum and palladium in hydrothermal solutions: A thermodynamic approach. Econ. Geol. 1988, 83, 492–510. [CrossRef] 36. Spiridonov, E. Ore-magmatic systems of the Noril’sk ore field. Russ. Geol. Geophys. 2010, 51, 1059–1077. [CrossRef] 37. Gorbachev, N.S.; Dadze, T.P.; Kashirtseva, G.A.; Kunts, A.F. Fluid transfer of gold, palladium, and rare earth elements and genesis of ore occurrences in the Subpolar Urals. Geol. Ore Deposits 2010, 52, 215–233. [CrossRef] 38. Olivo, R.; Gauthier, M.; Bardoux, M. Palladian gold from the Caue iron mine, Itabira District, Minas Gerais, Brazil. Mineral. Mag. 1994, 58, 579–587. [CrossRef] 39. Varajão, C.; Colin, F.; Vieillard, P.; Melfi, A.; Nahon, D. Early weathering of palladium gold under lateritic conditions, Maquiné Mine, Minas Gerais, Brazil. Appl. Geochem. 2000, 15, 245–263. [CrossRef] 40. Cabral, A.R.; Lehmann, B.; Kwitko, R.; Jones, R.D. Palladian gold and palladium arsenide-antimonide minerals from Gongo Soco iron ore mine, Quadrilátero Ferrífero, Minas Gerais, Brazil. Appl. Earth Sci. 2002, 111, 74–80. [CrossRef] Minerals 2021, 11, 451 25 of 26

41. Chapman, R.J.; Leake, R.C.; Bond, D.P.G.; Stedra, V.; Fairgrieve, B. Chemical and Mineralogical Signatures of Gold Formed in Oxidizing Chloride Hydrothermal Systems and their Significance within Populations of Placer Gold Grains Collected during Reconnaissance. Econ. Geol. 2009, 104, 563–585. [CrossRef] 42. Chudnenko, K.V.; Palyanova, G.A.; Anisimova, G.S.; Moskvitin, S.G. Ag-Au-Hg solid solutions and physicochemical models of their formation in nature (Kyuchyus deposit as an example). Appl. Geochem. 2015, 55, 138–151. [CrossRef] 43. Palyanova, G.; Murzin, V.; Zhuravkova, T.; Varlamov, D. Au-Cu-Ag mineralization in rodingites and nephritoids of the Agardag ultramafic massif (southern Tuva, Russia). Russ. Geol. Geophys. 2018, 59, 238–256. [CrossRef] 44. Nielsen, T.F.D.; Andersen, J.C.O.; Holness, M.; Keiding, J.K.; Rudashevsky, N.S.; Rudashevsky, V.N.; Salmonsen, L.P.; Tegner, C.; Veksler, I.V. The Skaergaard PGE and Gold Deposit: The Result ofin situFractionation, Sulphide Saturation and Magma Chamber-scale Precious Metal Redistribution by Immiscible Ferich Melt. J. Pet. 2015, 56, 1643–1676. [CrossRef] 45. Rudashevsky, N.S.; Rudashevsky, V.N.; Nielsen, T.F.D.; Shebanov, A.D. Alloys and intermetallic compounds of gold and copper in gold-palladium ores of the Skaergard massif (Greenland). J. Proc. Komi Sci. Centre Ural Branch RAS 2014, 143, 1–23. 46. Sluzhenikin, S.F.; Mokhov, A.V. Gold and silver in PGE-Cu-Ni and PGE ores of the Noril’sk deposits, Russia. Miner. Depos. 2014, 50, 465–492. [CrossRef] 47. Palacios, C.; Hérail, G.; Townley, B.; Maksaev, V.; Sepulveda, F.; De Parseval, P.; Rivas, P.; Lahsen, A.; Parada, M.A. The composition of gold in the cerro casale gold-rich porphyry deposit, maricunga belt, Northern Chile. Can. Miner. 2001, 39, 907–915. [CrossRef] 48. Arif, J.; Baker, T. Gold paragenesis and chemistry at Batu Hijau, Indoneisa: Implications for gold-rich porphyry copper deposits. Miner. Depos. 2004, 39, 523–535. [CrossRef] 49. Chudnenko, K.; Pal’yanova, G. Thermodynamic properties of solid solutions in the Ag-Au-Cu system. Russ. Geol. Geophys. 2014, 55, 349–360. [CrossRef] 50. Gas’kov, I.V. Major impurity elements in native gold and their association with gold mineralization settings in deposits of Asian fold belts. Russ. Geol. Geophys. 2017, 58, 1080–1092. [CrossRef] 51. Murzin, V.V.; Sustavov, S.G. Solid-phase transformation in natural cuprous gold. Izv. USSR Acad. Sci. Ser. Geol. 1989, 11, 94–104. (In Russian) 52. Okamoto, H.; Charkrabarti, D.J.; Laugylin, D.E.; Massalski, T.B. The Au-Cu (Gold-Copper) System. Bull. Alloy Ph. Diagr. 1987, 8, 454–473. [CrossRef] 53. Spiridonov, E.M.; Pletnev, P.A. Zolotaya Gora Cupriferous Gold Deposit “Gold-Rodingite” Formation; Spiridonov, E.M., Ed.; Nauchnyi Mir: Moscow, Russia, 2002; p. 220. (In Russian) 54. Murzin, V.V.; Chudnenko, K.V.; Palyanova, G.A.; Varlamov, D.A.; Naumov, E.A.; Pirajno, F. Physicochemical model of formation of Cu-Ag-Au-Hg solid solutions and intermetallic alloys in the rodingites of the Zolotaya Gora gold deposit (Urals, Russia). Ore Geol. Rev. 2018, 93, 81–97. [CrossRef] 55. Knight, J.; Leitch, C.H. Phase relations in the system Au-Cu-Ag at low temperatures, based on natural Assemblages. Can. Miner. 2001, 39, 889–905. [CrossRef] 56. Surenkov, S.V.; Moralev, G.V.; Borisov, A.V. Physicochemical Parameters of the Au-PGE-REE Mineralization of the Chudnoe and Nesterovskoe Ore Occurrences (Subpolar Urals); VII Student School Metallogeny of Ancient and Modern Oceans-2001; Institute of Mineralogy, Ural Branch of RAS: Miass, Russia, 2001; pp. 195–198. (In Russian with English abstract) 57. Omelyanenko, B.I. Near-Ore Hydrothermal Alteration of Rocks; Nedra: Moscow, Russia, 1978; p. 215. (In Russian) 58. Prokofiev, V.Y. Geochemical Features of Ore-Forming Fluids of Hydrothermal Gold Deposits of Various Genetic Types (According to the Study of Fluid Inclusions); Zorina, L.D., Ed.; Siberian Publishing Company “Science”: Novosibirsk, Russia, 2000; p. 192. 59. Berni, G.V.; Heinrich, C.A.; Wälle, M.; Wall, V.J. Fluid geochemistry of the Serra Pelada Au-Pd-Pt deposit, Carajás, Brazil: Exceptional metal enrichment caused by deep reaching hydrothermal oxidation. Ore Geol. Rev. 2019, 111, 102991. [CrossRef] 60. Barakat, A.; Marignac, C.; Boiron, M.-C.; Bouabdelli, M. Caractérisation des paragenèses et des paléocirculations fluides dans l’indice d’or de Bleïda (Anti-Atlas, Maroc). Comptes Rendus Geosci. 2002, 334, 35–41. [CrossRef] 61. El Ghorfi, M.; Oberthür, T.; Melcher, F.; Lüders, V.; Boukhari, A.; Maacha, L.; Ziadi, R.; Baoutoul, H. Gold–palladium mineralization at Bleïda Far West, Bou Azzer–El Graara Inlier, Anti-Atlas, Morocco. Miner. Depos. 2006, 41, 549–564. [CrossRef] 62. Berni, G.V.; Heinrich, C.A.; Lobato, L.M.; Wall, V. Ore mineralogy of the Serra Pelada Au-Pd-Pt deposit, Carajás, Brazil and implications for ore-forming processes. Miner. Deposita 2016, 51, 781–795. [CrossRef] 63. Palyanova, G.A. Gold and Silver Minerals in Sulfide Ore. Geol. Ore Depos. 2020, 62, 383–406. [CrossRef] 64. Zaccarini, F.; Pushkarev, E.; Fershtater, G.B.; Garuti, G. Composition and mineralogy of PGE-rich chromitites in the nurali lherzolite gabbro complex, Southern Urals, Russia. Can. Miner. 2004, 42, 545–562. [CrossRef] 65. Anikina, E.V.; Zaccarini, F.; Knauf, V.V.; Rusin, I.A.; Pushkarev, E.V.; Garouti, J. Palladium and gold minerals in the ores of the Baron ore occurrence, Volkovsky gabbro-diorite massif. Bull. Ural Dep. Rus. Mineral. Soc. 2005, 4, 5–25. (In Russian with English abstract) 66. Spiridonov, E.; Yanakieva, D. Modern mineralogy of gold: Overview and new data. ArchéoSciences 2009, 33, 67–73. [CrossRef] 67. Kuznetsov, S.K.; Tarbaev, M.B.; Moralev, G.V.; Soboleva, A.A.; Ivanova, T.I. Gold-platinoid mineralization in the Subpolar Urals. In Mater. Vseros. Conf. “Gold, Platinum and Diamonds of the Komi Republic and Adjacent Regions”; Institute of Geology, Komi Science Center: Syktyvkar, Russia, 1998; pp. 13–14. (In Russian with English abstract) Minerals 2021, 11, 451 26 of 26

68. Buisson, G.; Leblanc, M. Gold bearing listwanites (carbonatized ultramafic rocks) in ophiolite complexes. In Metallogeny of Basic and Ultrabasic Rocks; Gallagher, J.M., Iscer, R.A., Neary, C.R., Prichard, H.M., Eds.; Institute of Mining and Metallurgy: London, UK, 1986; pp. 121–132. 69. Halls, C.; Zhao, R. Listvenite and related rocks: Perspectives on terminology and mineralogy with reference to an occurrence at Cregganbaum, Co., Mayo, Republic of Ireland. Miner. Depos. 1995, 30, 303–313. [CrossRef] 70. Azer, M.K. Evolution and economic significance of listwaenites associated with Neoproterozoic ophiolites in south Eastern Desert, Egypt. Geol. Acta 2013, 11, 113–128. 71. Belogub, E.V.; Melekestseva, I.Y.; Novoselov, K.A.; Zabotina, M.V.; Tret’Yakov, G.A.; Zaykov, V.V.; Yuminov, A.M. Listvenite- related gold deposits of the South Urals (Russia): A review. Ore Geol. Rev. 2017, 85, 247–270. [CrossRef] 72. Sazonov, V.N. Gold-Bearing Metasomatic Associations in Fold Belts; Institute of Geology and Geochemistry: Yekaterinburg, Russia, 1998; p. 181. (In Russian) 73. Zharikov, V.A.; Rusinov, V.L. Metasomatism and Metasomatic Rocks; Scientific World: Moscow, Russia, 1998; p. 492. 74. Harlov, D.E.; Austrheim, H. Metasomatism and the Chemical Transformation of Rock; Metzler, J.B., Ed.; Springer: Berlin/Heidelberg, Germany, 2013; p. 806.